Abstract: A multi-purpose locking device including a key made of a material, which is not a metal, but a synthetic resin, thereby enabling the key to be easily carried by the user. The key also has a configuration capable of selecting a key number from numerous possible key numbers, thereby preventing even skilled lock-picking persons from opening the locking device unless the key is used. The key includes a plurality of fine apertures and block members fitted in selected ones of the apertures and adapted to close those apertures. Light receiving positions are set by the apertures not closed by the block members whereas light shielding positions are set by the apertures closed by the block members. A locked state of the locking device is unlocked only when light is received at the light receiving positions while being shielded at the light shielding positions.