To easily mount two rim lock cases, for example, to the internal wall surface of a door in a fine looking state and to dispose a long-sized indoor handle to correspond to these lock cases.

A rim lock case mounting structure includes a mounting base fixed to the internal wall surface of the door. The mounting base includes at least a first base part fixed to the internal wall surface, and a second base part orthogonal to the first base part. A long seat having a longitudinal internal space is fixed to the second base part through a first notched mounting part formed on the door end side so that the second base part is flush with the outer surface of one sidewall. The one sidewall is formed with a plurality of longitudinal openings and second notched mounting parts at the upper and lower ends of the longitudinal openings respectively. The upper and lower rim lock cases built into the longitudinal internal space are fixed so that the respective front parts are flush with the outer surface of the one sidewall.