To provide an auxiliary lock device which can be mounted on any type of door through a simple operation without the use of a tool, and which can acquire sufficient mounting strength.

This auxiliary lock device, which can be demountably mounted on the door making an opening/closing motion with respect to a door frame, comprises an outdoor portion, an indoor portion, and a connecting portion for integrally connecting the outdoor and indoor portions together in the state of sandwiching the door between them. The indoor portion comprises: an indoor-side fixed part which is fixed in the state of abutting on the indoor-side surface of the door in mounting; at least one shaft which is fixed to the indoor-side fixed part, and longitudinally elongated in the mounting; a locking mechanism which can move along the at least one shaft, and which can lock the door by being locked to the door frame in the mounting; and a longitudinal lock mechanism which inhibits the locking mechanism from being moved along the at least one shaft through a single operation.