Itagaki Nobuyasu served as a bushō during the Sengoku period.  He was a retainer of the Kai-Takeda clan.

Nobuyasu descended from the Ozo clan, a branch of the Kai-Genji.  From the eighth month of 1557, Itagaki Nobunori, the lineal heir of Itagaki Nobukata, was removed from his position by Takeda Harunobu (Shingen) on grounds of misconduct, so the Itagaki family came at once to an end.

In 1558, Nobukata’s son-in-law, Ozo Nobuyasu, was ordered by Shingen to inherit the family name of the Itagaki and revive the clan.  In a written pledge from 1567, he used the Itagaki surname and had 120 soldiers comprised of relatives of the Takeda clan.  According to historical accounts of Kai Province, he donated land for the Kandaten Shrine in Ozo and the Ozo estate.

In 1569, during the Invasion of Suruga, Nobuyasu served as the lord of Kunō Castle in Suruga.  He was succeeded by Imafuku Tomokiyo.  After the death in battle of Naitō Masahide, Nobuyasu served on an interim basis until the appointment of Naitō Masaaki in the second month of 1579 as the next chamberlain of Minowa Castle in Kōzuke Province.

In the first month of 1570, during an assault on Hanazawa Castle, the Takeda forces laid siege to Toku-no-isshiki Castle (controlled by the Imagawa clan).  After the capture of Toku-no-isshiki, Takeda Shingen ordered the castle to be renovated and renamed Tanaka Castle.  Yamagata Masakage, one of the Four Guardian Kings of the Takeda, served as lord of the castle, but, in 1572, after the Takeda forces commenced the Western Expedition, Shingen replaced him with Nobuyasu and, in 1573, built Suwahara Castle in the Haibara District of Tōtōmi Province to strengthen his position vis-à-vis the Tokugawa clan.

Nobuyasu, as the lord of Tanaka Castle, kept cavalry horses and was called Lord Itagaki of Tanaka.  At the Fuji-Omuro Sengen Shrine in Yamanashi Prefecture and the Ikushima-Tarushima Shrine in Nagano Prefecture, written pledges and documents with personal seals have been passed down for generations.

In the midst of the Western Expedition, Shingen died of illness and, in 1575, after Takeda Katsuyori suffered a major defeat at the Battle of Nagashino, the Tokugawa clan captured Suwabara Castle while auxiliary castles to Tanaka Castle also fell, resulting in its isolation.

In 1582, Anayama Nobutada (Baisetsu), a relative of the Takeda and lord of Ejiri Castle, betrayed the Takeda in favor of the Tokugawa.  This led Yoda Nobushige, the lord of Tanaka Castle, to vacate the premises so it fell to the Tokugawa.

On 5/21 of Tenshō 3 (1575), after Naitō Masahide, the chamberlain of Minowa Castle in Kōzuke Province was killed at the Battle of Nagashino, Nobuyasu resided in the castle.  Nobuyasu was accompanied by his lineal heir, Itagaki Surinosuke.  Takeda Katsuyori entrusted the governance of Kōzuke to Sanada Masayuki (the chamberlain of Shiroi Castle) and Nobuyasu as the chamberlain of Minowa Castle.

After the decimation of the Takeda clan by the Oda army in the third month of 1582, his son, Surinosuke, served Sanada Masayuki.  In 1585, at the First Battle of Ueda in Shinano Province, Surinosuke fought valiantly.