Following the end of warm-up, a control unit confirms whether the warm-up time is 10 seconds or more. If the warm-up time is 10 seconds or more, the control unit controls a maximum lamp turn-on time for alternate turn-on at 1.0 second as long as an idle time (t) from the end of warm-up is less than two minutes. When the idle time exceeds two minutes, the control unit controls the maximum lamp turn-on time for alternate turn-on at 2.5 seconds.