1. A collision severity determining system for determining severity of a collision of a vehicle, comprising: a first deceleration detector disposed substantially in a central portion of a body of the vehicle so as to detect a vehicle deceleration in a longitudinal direction of the vehicle; a second deceleration detector disposed in a front portion of the vehicle that is located ahead of the first deceleration detector, so as to detect a deceleration at the front portion in the longitudinal direction of the vehicle; a velocity change amount calculating unit that calculates a velocity change amount of the vehicle by integrating the vehicle deceleration detected by the first deceleration detector with respect to time; a severity determining unit storing two or more severity determination maps defined by the vehicle deceleration detected by the first deceleration detector and the velocity change amount, each of the severity determination maps representing a threshold pattern consisting of threshold values with which the vehicle deceleration is compared so as to determine severity of the collision of the vehicle; and a problem detecting unit that generates a problem detection signal that indicates that a problem has occurred to the vehicle, when detecting an absence of a signal indicative of the deceleration transmitted from the second deceleration detector, wherein when receiving the problem detection signal from the problem detecting unit, the severity determining unit operates in a predetermined fail-safe mode for selecting an appropriate one of the two or more severity determination maps, and determines severity of the collision of the vehicle, based on the selected severity determination map.