<p>A small-sized acceleration sensor which senses a state of a sudden deceleration of a vehicle so as to actuate an actuator such as an air bag. The acceleration sensor comprises an ignition pin, an inertial mass body inertially moving from an original position when acceleration reaches a predetermined value, urging means for urging the ignition pin in a direction in which the ignition pin strikes a detonating material, and trigger means which prevents an urging force of the urging means from being applied to the ignition pin and which transmits the urging force of the urging means to the ignition pin during the movement of the inertial mass body. The ignition pin is disposed at a rear side of the inertial mass body in a direction opposite to the moving direction of the inertial mass body. <IMAGE></p>
