A safing decision is executed using a first integrator for integrating acceleration sensor output values for a first integration interval to obtain a first arithmetic operation value, a second integrator for cumulative integration of the acceleration sensor output values using it as a trigger that the first arithmetic operation value exceeds a first predetermined value to obtain a second arithmetic operation value, a third integrator for integrating the second arithmetic operation value for a third integration interval to obtain a third arithmetic operation value as a second order integration value of the output values of the acceleration sensors, and a comparator for comparing a map derivation value derived by substituting the third arithmetic operation value into a threshold value map and the first arithmetic operation value with each other. The airbag is started up when the main decision and the safing decision are carried out as an on decision.