An unlocking device includes a casing mounted to a side of a lock having a keyhole. A movable body is rotatably received in the casing about an axis. The movable body has a closed end having a through-hole. A key receiving seat is received in the movable body and has a passage. The keyhole is normally blocked by the closed end of the movable body. The movable body can be rotated to align the through-hole with the keyhole and the passage. A manually rotatable linking rod extends through the passage and has an engaging portion for coupling a key received in the passage. The key can be moved into the keyhole by moving the linking rod along the axis and can be rotated for a locking or unlocking operation.