It is an object of the present invention to provide a collision detector for a vehicle, capable of avoiding the adverse effect of the diagnostic pulses on the detection of a collision so that the failure of the acceleration sensor can be diagnosed correctly. A collision detector in accordance with the present invention comprises a diagnostic pulse generator which applies diagnostic pulses to an acceleration sensor to detect the failure of the acceleration sensor, and a diagnostic circuit which stops the output diagnostic pulses of the diagnostic pulse generator when the output of the acceleration sensor exceeds a malfunction avoidance level which is lower than a reference collision level.