Abstract: A person-on-board protecting apparatus having ignition timing estimating means for outputting an ignition signal by determining a collision on the basis of an output from an acceleration sensor and estimating timing at which the head of a person on board reaches a predetermined position, expansion rate control means for outputting a control signal for controlling the expansion force of an air bag according to the head position of the person on board from a forward predetermined reference position, and an inflator having a plurality of detonators. The ignition modes of the plurality of detonators provided for the inflator are controlled according to an ignition signal from the ignition timing estimating means and a control signal from expansion force control means, so that the expansion rate and timing of the air bag are controlled.