To provide an auxiliary lock for a window, applicable whether a new building or an existing building and used with a crescent lock to be independently locked and unlocked.

This auxiliary lock 10 is fitted adjacent to the crescent lock 1 mounted on the style 51 of an inside window sash, and locked and unlocked by a remote control device. The auxiliary lock 10 is formed by a flat base plate 26 and an auxiliary lock body 11 having a locking member. The base plate 26 is fitted with a fitting screw 6 of the crescent lock 1 using a fitting screw hole of the crescent lock 1 mounted on the style 51 of the inside window sash. A hook member 30 is provided with a first hook part 31 where a cam 3 of the crescent lock 1 is engaged and a second hook part 32 where a locking member of the lock body 11 is engaged, and fitted with a fitting screw hole of the hook member mounted on a style 53 of an outside window sash.