A hole 13 though which a firing pin 6a of a firing lever 6 is projected outside a housing 1 is formed in a wall 1c of the housing which is in parallel with a direction of an external shock applied to the housing 1. This structure ensures the rotational angle of the firing lever 6 and a biasing force of a spring 9 which urges the firing lever 6 can be used effectively as kinetic energy thereof.
