To improve mountability and mounting strength of a sub lock in a resin sash.

A resin sash comprises a frame 1 composed of an upper frame 1a and a lower frame 1b as well as a right vertical frame 1c and a left vertical frame 1d, sashes 2, 3 at least one of which is provided movably openwise/ closewise, and a sub block 12 abuts against the inner sash 2 to thereby restrain openwise movement of the sashes 2, 3. The sub lock 12 has a case 13 and a lock body 14 accomodated in the case 13 and can project the inner side of a room. Engaging grooves 45, 46 engaging with a circumferential part of a mounting hole 37 formed at the room inner side of the outer sash 3 are formed in a lift and fit style on both longitudinal ends of the case 13. Opposite edges of one of engaging grooves 45 are formed as a swelling or extending parts 40, 41 extending in the longitudinal direction. The parts 40, 41 are fixed by a fixing means 39, with the circumferential part of a mounting hole 37 held between them 40, 41.