To provide a auxiliary lock not easily removed from the outside, not requiring work of making holes in a sash, maintaining a locking effect even if a sub-lock is installed and having no possibility of being inhibited from locking and unlocking from the inside.

The auxiliary lock is used for locking an inside door 10 boring a hole in the end face 11 and an outside door 20 by having the sash assembled with a vertical material 1 and a horizontal material 2, and is constituted of a shaft 4 having one end side putting in and out of the hole 3, a spring 5 engaged with the shaft 4 and a body section 6, the front plate 64 wherein a slot 64a is bored to allow the spring 5 to move in the X direction, an axial movement section 9 of a shaft side spring bearing 42 enabling the shaft 4 to move in the X2 direction, an attachment section 8 fixed to a horizontal material surface 21 by making the body section possible to be attached and detached, etc. Locking can be easily and surely made by making use of the hole of the sash.