A mechanical acceleration sensor includes a sensor casing, a weight, a spring-loaded firing pin, and a latch lever, the weight, the firing pin, and the latch lever, each being movably mounted in the sensor casing in such a manner as to either translate along an axis or to rotate about an axis, the axes of translation of the members that translate being substantially perpendicular to that the axes of rotation of the members that rotate. The latch lever is loaded with a set load to engage the firing pin and is positioned to be engaged by the weight upon inertial movement of the weight to disengage from the firing pin, thereby allowing the firing pin to perform a percussive firing action by the spring load. The casing has an opening for receiving the weight, firing pin and latch lever, and shaft support grooves are provided in the casing such that each shaft support groove is open at an end thereof which faces the opening. A shaft for a member that rotates is supported by the shaft support grooves.