Abstract: An acceleration sensor is constructed of a spherical inertia element made of a magnetic material and having a predetermined mass, an annular magnet defining a retainer hole on which the inertia element is allowed to rest and is retained until a predetermined acceleration is applied thereto, a switch change-over member for changing over a switch in response to movement of the inertia element from the retainer hole onto an upper wall of the magnet upon application of the predetermined acceleration, a rod slidably inserted in the retainer hole, and a coil spring biasing the rod toward the inertia element. When the inertia element has jumped out from the retainer hole onto an upper wall of the magnet, the rod is caused to move upward by the biasing force of the coil spring so that a free end portion of the rod closes the retainer hole.