pulses each having a pulse width equal to one period of the clock signal clk via its terminals m 1 m 2 m n as a result the transistors b 1 b 2 b n are sequentially kept on for one period of the clock signal clk one after another specifically in such a way that the transistor b 1 remains on between time points t 4 and t 6 the transistor b 2 remains on between time points t 6 and t 8 and so forth thus the image reading photodiodes pd 1 pd 2 pd n are initialized sequentially in this way if one pays attention to the first image reading photodiode pd 1 whereas the transistor c 1 is kept on between time points t 3 and t 5 so that the signal of the photodiode pd 1 is read out the transistor b 1 is also kept on between time points t 4 and t 5 i e during the last half of the read period so that the photodiode pd 1 is initialized thus only the signal read out between time points t 3 and t 4 i e during the first half of the read period is actually used likewise also with the other image reading photodiodes pd 2 pd 3 pd n only the signals read out during the first half of their respective read period are actually used moreover as fig2 shows the shift register 3 also sequentially outputs in synchronism with the trailing edges of the clock signal clk after.