To improve operability of an auxiliary lock device for sliding door by facilitating switching of a lock member of an unlocked attitude to a locked attitude.

The auxiliary lock device for sliding door comprises a mounting base 5 attachable on the indoor side surface of an outdoor sliding door 2, and a lock member 6 provided on the mounting base 5 so as to be vertically rockable, the lock member 6 being switchable to the unlocked attitude where it is deviated from a traveling route of an indoor sliding door 3 and the locked attitude where it is protruded to the traveling route of the indoor sliding door 3. The device further comprises an energizing member 9 for regularly energizing the lock member 6 to the lock attitude side, a retaining member 12 for retaining the lock member 6 in the unlocked attitude against the energizing member 5, and an operation member 13 for releasing the retention by the retaining member 12.