A collision severity determining system for determining severity of a collision of a vehicle includes a first deceleration detector disposed in a central portion of a vehicle for detecting a deceleration in a longitudinal direction of the vehicle, and a second deceleration detector disposed in a front portion of the vehicle for detecting a deceleration in the longitudinal direction. A controller calculates a velocity change amount of the vehicle by integrating the vehicle deceleration with respect to time, and determines severity of the collision by comparing the vehicle deceleration with a threshold value corresponding to the same velocity change amount. When a problem, such as a failure or breakdown of the second deceleration detector, occurs, the severity determining unit operates in a predetermined fail-safe mode for selecting an appropriate severity determination map stored in the controller, and makes a severity determination based on the selected severity determination map.