Ayugase Sanemitsu served as a bushō during the Sengoku period. He was a retainer of the Iono clan who were one of the key families affiliated with the Nasa clan of Shimotsuke Province known as the Seven Clans of the Nasu.
While serving the Iono clan, Sanemitsu contributed with valor in battles across the province. He was known as an expert archer.
On 9/17 of Tenbun 18 (1549), Nasu Takasuke led 300 soldiers to invade the territory of the Utsunomiya. As a result, upon orders from Ashikaga Haruuji, the Koga kubō, Utsunomiya Hisatsuna led an army of 2,500 men to deploy to the Kitsure River and engage in battle against the Nasu forces.
Initially, the Utsunomiya enjoyed an advantage based on their superior numbers, so Takasuke used troops lying in ambush in Sōtomezaka to create havoc for the Utsunomiya army, reversing the tide of the battle in favor of the Nasu. Takō Nagatomo (a retainer of the Utsunomiya), Michikawa Tadachika (a retainer of the Kasama), and the Yokota brothers (five sons of Yokota Tsunamura who were retainers of the Yokota) fought valiantly, halting the assault by the Nasu army, but Tadachika and the Yokota brothers were killed in action. These individuals are known as the Five Brothers of the Yokota.
In an effort to control his army amidst the chaos, Hisatsuna went to the front lines of the battle but died after being struck in the chest by an arrow shot by Sanemitsu. The battle ended in victory for the Nasu army led by Takasuke. As recognition for his achievement, the head of the Iono clan, Iono Sukemune, awarded Sanemitsu the honorary title of Bungo-no-kami. Thereafter, he spent ten Yongle Tongbao (a Ming dynasty era Chinese cash coin produced under the reign of the Yongle Emperor imported to Japan from the era of Ashikaga Yoshimitsu) and, in Sōtomezaka, constructed a stone memorial for Hisatsuna. For this reason, this location is referred to as jūkan-yagorō-zaka, or the slope where Yagorō invested 10 coins.