A light emitter driving device has a decoder portion which monitors a rectified voltage and generates a dimming signal, and a drive current control portion which controls a drive current to a light emitter according to the dimming signal. The decoder portion has a comparator which compares the rectified voltage with a predetermined threshold voltage to generate a comparison signal, a sampling counter which measures high-level and low-level periods of the comparison signal, a duty calculation portion which calculates the duty of the rectified voltage based on the output of the sampling counter, a filter calculation portion which excludes sporadic variation in duty by applying digital filtering to the output of the duty calculation portion, and a dimming signal generation portion which generates the dimming signal based on the output of the filter calculation portion.