His main name was SHIMOTSUKENU no Ason (second highest of the eight hereditary titles), however, according to Shoku Nihongi (Chronicle of Japan Continued), on May 2, 707, SHIMOTSUKENU no Komaro, a member of his family, asked the Imperial Court if SHIMOTSUKENU no Iwashiro could change Iwashiro's family name into 'SHIMOTSUKENU no Kawachi Ason' from 'SHIMOTSUKENU no Ason' and received permission for it, therefore it is believed that Iwashiro changed his family name into SHIMOTSUKENU no Kawachi Ason at this point.
本姓は下毛野朝臣だが、続日本紀によると707年（慶雲4年）3月22日(旧暦)、一族の下毛野古麻呂が石代の姓を「下毛野朝臣」から「下毛野川内朝臣」に変更することを朝廷に上申し、許可が下りていることから、この時点で下毛野川内朝臣に改姓したものと考えられる。