1. An offset adjusting device comprising: a sample-hold unit that sample-holds an analog image signal obtained by converting reflected light from an original into an electric signal with a photoelectric conversion device to thereby obtain a sample-hold signal; an amplifying unit that amplifies the sample-hold signal to obtain an amplified signal; an analog-digital converter that digitizes the amplified signal to obtain a digital signal; a difference detecting unit that detects a difference between a black-level detection value of the digital signal and a black-level target value; an selecting unit that selects an adjustment coefficient among a plurality of adjustment coefficients based on comparison of the difference and a reference value; and a feedback unit that subjects an offset adjustment value based on the adjustment coefficient selected by the selecting unit to feedback processing.