A collision sensing system for vehicles is provided in which a moving core shaft having alternately notched and projecting portions formed thereon or a permanent magnet serving as a moving core and magnetized to create therein alternate north and south poles is disposed to move relative to a stator for providing a magnetic path, whereby when a vehicle comes into collision, the magnetic reluctance of the magnetic path is caused to change rapidly at plural times and a generating winding wound on the stator thus generates the waveform of an output voltage proportional to the rate of the plurality of changes of the magnetic reluctance. This output voltage is compared with a predetermined value to determine, whether a controlled system such as an air bag should be actuated to thereby actuate the controlled system properly.