To provide a sliding door auxiliary lock easy to handle with simple locking/unlocking operation and variable in the locking/unlocking cord.

The sliding door auxiliary lock comprises a lock receiver 3 to be fixed to a sliding door 2 movable relative to the other member 1 with opening/closing operation and a lock device 4 fixed to the other member 1 for prohibiting the relative movement of the sliding door 2 in a locking condition. The locking device 4 has a plurality of operation members 5 movable between a locking position and an unlocking position for engaging with the lock receiver 3 with the energization to the locking position. In the operation members 5, there are mixed a proper number of locking portions 6 to be moved to the unlocking position with the tensile operation from the locking device 4 and a proper number of dummy members 8 to be moved to the locking position with component force that a face 7 riding on the lock receiver 3 receives with the operating force of the sliding door 2 in the opening direction.