In a system for warning a failure to wear a seat belt to urge a vehicle operator to wear the seat belt, a sound emitter is activated when a vehicle speed is higher than a threshold level and the seat belt is not fastened. The sound emitter cyclically alternates between a sound emitting state and a silent state when activated, the silent state lasting for a period of at least three seconds each time. Because the silent state is interposed between sound emitting states and because the warning is not emitted when traveling for a short period of time at a low speed, for instance, in a parking garage or the like or when the vehicle is stationary, the annoyance to the vehicle operator is minimized while allowing the vehicle operator to be kept warned.