To provide a lock mounting device capable of easily mounting a lock to a door such as a gate door heretofore considered difficult to mount a lock case, to attain a waterproofing effect at least on the lock, to prevent parts from rusting inside the lock and to eliminate an operation failure or the like.

The door 1 such as the gate door to which engraving or surface attachment of the lock case cannot be carried out, is provided with an auxiliary member 7, and the auxiliary member 7 is provided with a lock box 20. The lock case 30 is mounted into the lock box 20, and a column or a frame 4 which the door such as the gate door faces, is provided with a lock receiving box 50 facing the lock box 20. A receiving member 40 having a receiving hole 42 for engaging a lock rod 32 provided at the lock case 30, so as to retractably put in and out freely, is mounted into the lock receiving box 50.