A method for preventing dozing off, comprising the steps of: a measurement step measuring human heartbeats; a decision step (S4 to S8) generating a trigger, hereinafter referred to as a “drowsiness trigger”, for indicating occurrence of drowsiness when a state in which a time interval between adjacent heartbeats, hereinafter referred to as a “heartbeat interval”, is longer than the previous heartbeat interval consecutively occurs; and a warning step (S9) giving a warning to prevent dozing off when the drowsiness trigger is generated.