PURPOSE: To dim a light source of lighting and to obtain a photoelectric conversion signal with an always constant level, by comparing a previously set up reference color level with a reading picture element in a color part corresponding to the reference color level, and controlling the lighting period of a lighting signal inputted to a dimming circuit in accordance with the compared result.

CONSTITUTION: A circuit 9 is a dimming circuit of a fluorescent lamp to which AC power is supplied, a unit 10 is a white coating part to obtain a reference reading signal which is fitted on an original holding member for a reading original 3, and a circuit 11 is a comparator to compare a picture signal from the white coating part 10 which is taken out from an A/D converter 7 with a white reference picture signal and generates a compared output when the reference reading picture signal is larger than the white reference picture signal. In said constitution, a comparator 12 compares the variation of a photoelectric conversion signal level of the white coating part 10 due to the change of the quantity of light of a fluorescent part 2 with the white reference picture signal and the dimming circuit 9 controls the quantity of light of the fluorescent lamp 2 in accordance with the output of the comparator 11.