To firmly fit and fix an anchor pin by forming the head section of the anchor pin driven into the through hole of a caisson mat larger in size than one mesh of the wire net of the caisson mat.

Counterbore holes are formed concentrically with multiple through holes on one side face, and a metal net 23 is internally arranged on the other side face opposite to the counterbore holes. The metal net 23 is arranged with many rod-like steels 23a vertically and horizontally into a cross shape, and the metal net 23 is assembled into a caisson mat and integrally molded. The insertion shaft section 31 of a mat fitting tool provided with a lock head section 32 having the area larger than one mesh of the metal net 23 is inserted into the through hole from the counterbore hole side and lock on the caisson mat. The insertion shaft section 31 is buried, solidified, fitted, and fixed integrally with caisson molding concrete when a caisson is molded.