Originally, Tokimoto's family name was KAJI, but he succeeded the Isobe clan since the family line of Isobe ended when Kagehide ISOBE died, a grandson of Hidetada ISOBE, Tokimoto's elder brother.
もともとは加地氏を名乗っていたが、兄の磯部秀忠の孫、景秀の代で断絶した為に磯部氏を継承した。