To provide a power supply lock pin capable of preventing misoperation with a power supply switch when a part of operation prohibition tags are taken away among a plurality of operation prohibition tags.

A power supply lock pin 3 is used with a power supply lock device in which an engagement member 11 provided to a rotary operation lever 2a of a power supply switch 2 for reciprocal movement is caused to advance under such condition as a power supply is off, to regulate rotation of the rotary operation lever 2a. The power supply lock pin 3 is equipped with a lock part 31 which regulates receding of an engagement member 2b when sandwiched between the engagement member 11 and a rotary operation lever 2a, under such condition as inserted in an engagement hole 12 formed between the engagement member 11 which has advanced and the rotary operation lever 2a. The lower portion of a lock 31 is provided with a hooking part 34 which detachably hooks on an annular string member 5 that is separately attached to a plurality of operation prohibition tags 4, for suspending the plurality of operation prohibition tags 4.