His eldest son Yoshimasa married Sumie, who was the second daughter of a historian Taira SHIDEHARA, and his second son Yoshishige KOZAI is known as a Marxian philosopher.
長男・由正は歴史家・幣原坦の次女・澄江と結婚し、次男・古在由重はマルクス主義哲学者として知られている。