An action counter (27) counts the number of turn-on actions by an operator to an ignition switch (3) over a prescribed period of time, and a deceleration sensor (9) senses a deceleration imparted to the vehicle within a prescribed period of time. After the counted number of turn-on actions has exceeded a prescribed count value, if the deceleration exceeds a prescribed value, a switching intent determiner (5) makes a judgment of the operator's intent to switch the display mode of an air bag failure display device (17) from a normal mode to a maintenance mode.