To provide an anchor re-tensioning system and an anchor re-tensioning method for easily mounting a coupler without being affected by any foreign matter adhered to an anchor head.

A re-tensioning anchor system for re-tensioning an anchor installed underground includes: an anchor head formed with a circumferential groove on a side face at the end of the anchor; and a coupler having a cylindrically structured body section and an engagement section which is engaged with the anchor head in the inner periphery of the bottom of the body section at the anchor side. The coupler is configured so as to be separated into two semi-cylindrical pieces, and when the semi-cylindrical pieces are fitted into each other, each of the engagement sections of the semi-cylindrical pieces is engaged with the circumferential groove of the anchor head.