Naitō Masatoyo served as a bushō during the Sengoku period.  He is counted among the Four Guardian Kings of the Takeda.  His real name was established to be Naitō Masahide.

In 1522, Masatoyo was born as the second son of Kudō Toratoyo (Shimōsa-no-kami), a senior retainer of Takeda Nobutora.  The activities of Toratoyo are uncertain but, in 1508, he revolted against Takeda Nobutora and was defeated.  An individual referred to as Kudō-dono, together with Oyamada Hiramitsu of the Gunnai area (eastern portion of Kai Province), fled with the assistance of Ise Sōzui (Hōjō Sōun) of Nirayama in Izu Province.  This person is surmised to be Toratoyo.

Masatoyo first appears in historical records on 6/12 of Eiroku 2 (1559) in which he used the name of Kudō Genzaemon Taii nand served as a close associate of Shingen.  From around 1563 to 1570, Masatoyo defended Fukashi Castle in Shinano Province.  According to the account known as the Kōyō-gunkan, at the Fourth Battle of Kawanakajima in 1561, he affiliated with the main division and, as the general of a detached unit, led an attack from behind against the Uesugi army.

Until 1566, he served as the chamberlain of Fukashi Castle.  The Takeda clan invaded western Kōzuke, and, in the ninth month of the same year, captured Minowa Castle.  In the third month of 1567, the Takeda captured Shiroi Castle and annexed western Kōzuke.  From around 1566, Masatoyo served as a toritsugi, or intermediary, with those from Kōzuke including the Gokan clan.

From 1566 to 1567, the role of chamberlain of Minowa Castle was held by Kasuga Toratsuna and Sanada Yukitsuna and, from 1568 to 1569, Asari Nonbutane.  On 10/6 of Eiroku 12 (1569), Nobutane was killed at the Battle of Mimasetōge.  At this battle, Masatoyo led a supply division and replenished his own forces.  From around the fourth month of 1570, Masatoyo succeeded Nobutane as the chamberlain of Minowa.  He continued serving in this role until his death at the Battle of Nagashino in the fifth month of 1575.

As of 8/12 of Eiroku 10 (1567), he called himself Kudō Genzaemon-no-jō Masahide.  As of 8/26 of Eiroku 12 (1569), he called himself Naitō Surinosuke, adopting the surname of the Naitō family, hereditary retainers of the Takeda whose lineage had been extinguished.  At the same time, he is surmised to have taken the first name of Surinosuke.  Meanwhile, headship of the Kudō clan was adopted by his older brother, Masasuke.  In a letter from Takeda Shingen dated 4/3 of Eiroku 13 (1570), Masahide is referred to as Naitō Surinosuke.  On 4/10, he assumed his position as the chamberlain of Minowa Castle.  As a result, it is concluded that he changed to the Naitō surname at the same time that he became the chamberlain of the castle.

In the twelfth month of 1571, the Uesugi clan of Echigo Province made an offer via retainers (Kitajō Takahiro and Kitajō Kageie) for an alliance.  During this time, the Takeda entered into an alliance with Hōjō Ujimasa of Sagami Province.  After deliberating with a close associate of Shingen named Atobe Katsusuke, Masatoyo then rejected the overture from the Uesugi.

Later years

In the fourth month of 1573, after the death of Shingen, Masatoyo served his son, Takeda Katsuyori.

On 5/21 of Tenshō 3 (1575), at the Battle of Nagashino, Masatoyo, along with Hara Toratane and Yamagata Masakage, was positioned in the left wing and deemed to have joined with the forces from western Kōzuke.  The Naitō battalion fought against the main division of the Oda army.  According to one account, he clashed against Honda Tadakatsu, but it is uncertain.  As the Takeda forces withdrew in defeat, Masatoyo, together with Baba Nobuharu, stood their ground to enable Katsuyori time to escape.  Masatoyo was killed by Asahina Yasukatsu from the Tokugawa army when visiting Ieyasu’s camp as a messenger of Imagawa Ujizane.  He was fifty-four years old.

Prior to the Battle of Nagashino, Masahide changed his name to Yamato-no-kami.

Anecdotes

Masatoyo was known for being a skill military tactician and, together with Takeda Nobushige, was regarded at the rank of a lieutenant general of the Takeda army.  In the Kōyō-gunkan, Yamagata Masakage praised Masatoyo and Nobushige as worthy lieutenant generals.

Masatoyo participated in all of the major battles led by Shingen, consistently producing results on the battlefield.  He did not, however, receive any written commendations from Shingen.  According to the Kōyō-gunkan, Shingen praised him in noting that Surinosuke excelled in his skills as an archer, but, nevertheless, did not provide a written commendation.  Meanwhile, Masatoyo said “Battles are won by following the general’s strategy and it is trifling to focus on individual achievements.”  As such, he was not concerned about written commendations.  This was suggestive of the deep trust between Shingen and Masatoyo.

Descendants

After Masatoyo’s demise, he was succeeded by Naitō Masaaki (Senjirō).  Masaaki was the third son of Hoshina Masatoshi and later adopted by Masatoyo.  There is also a theory that Masaaki was a natural son of Masatoyo.  Other descendants included Naitō Tanetsugu and Naitō Masahiro, but there are no details regarding their achievements.

Naitō Konan, a scholar of East Asian history who lived during the early twentieth century, called himself a descendant of Naitō Masatoyo.