PURPOSE: To suppress fluctuation of LBS at the time of lighting, as well as the unlighting time, by imparting a hysteresis to the variation of phase angle data operated by an operating unit.

CONSTITUTION: A hysteresis unit 21 is fed with a data for setting the resolution of hysteresis and a new phase angle data θi from an operating unit 10. A phase angle data θout, i being outputted currently from the hysteresis unit 21, is stored in a memory 22 and delivered to a hysteresis unit 21. Since the hysteresis unit 21 outputs a current phase angle θi when the difference from the phase angle data θout, i stored in the memory 22 is higher than a preset resolution of hysteresis otherwise outputs the data θout, i stored in the memory 22, the fluctuation can be suppressed.