To provide a power supply cut-off device for ensuring easy resetting work when an electric system is circuited off because of the malfunction of an electrically mounting system.

A resetting mechanism 13 has a degassing hole 12 and a reset lever 14 concentrically provided on a shaft 7 so as to be inserted in the hole 12. When an electric system is cutoff by the shaft 7 moved with gas pressure, the reset lever 14 is extended to the outside of a cylinder head 6a through the hole 12 because the shaft 7 is arranged in the cylinder head 6a. On this account, the shaft 7 is moved to an initial position in a cylinder 6 by pushing the reset lever 14 extended to the outside of the cylinder head 6a to the inside of the cylinder head 6a, making it possible to put an input terminal 8 and an output terminal 11 into an electrically connected condition (cut-off cancelled condition).