A microcomputer 4 responds to an instruction signal to turn the lamp L on or off to control the FET 2 to be turned on or off through the drive circuit 5. The microcomputer 4 samples an electric current I L which is supplied through the A/D converter 6 and which flows in the lamp L at every predetermined sampling time T S and subjects the sampled electric current I L to a comparison with a known rated current value I R for the lamp L. When I L ≥ I R is satisfied, the microcomputer 4 starts counting energizing time T. When the accumulated value of I L × T S is enlarged to a predetermined value S O , the microcomputer 4 turns the FET 2 off. When the detected electric current I L is larger than an upper limit I O (> I R ) determined previously, the microcomputer 4 immediately turns the FET 2 off through the drive circuit 5.