Born in 1198, eldest son of the second shogun Minamoto no Yoriie. His mother was Wakasa no Tsubone, daughter of the powerful vassal Hiki Yoshikazu, and Ichiman bore the full hopes of the Hiki clan as the shogunal heir. In 1203, when Yoriie fell critically ill, the shogunate proposed dividing executive authority between Ichiman and his younger brother Sanetomo—effectively expanding Hiki influence. Hojo Tokimasa saw this as an intolerable threat. He lured Yoshikazu to the shogunal residence and had him killed, then immediately besieged the Hiki residence with a large force (the Hiki Rebellion). Ichiman, only 6 years old, reportedly perished as his maternal grandfather's compound was set ablaze. With the Hiki destroyed, Yoriie was deposed and confined at Shuzenji (killed the following year), and Ichiman's uncle Sanetomo was installed as the third shogun. The Hiki Rebellion—which cost an infant his life—is remembered as the bloody coup that definitively shaped the Kamakura Shogunate's power structure.

Killed at just 6 years old, there are few records of his personality. Yet his tragic death vividly illustrates the brutality of Kamakura power struggles.

The Hiki Rebellion was decisive in shaping the shogunate's power structure. Ichiman's death directly led to Yoriie's deposition and accelerated Hojo dominance.