An image reading apparatus includes a clamp-ratio controlling unit configured to change at least one of a clamp period and a line period for each read operation mode, wherein the clamp-ratio setting unit provides a control so that a clamp ratio that is a ratio between the clamp period and the line period is constant through all read operation modes. The clamp period is a period for performing clamp processing for setting a black level to a constant, which is an absolute black reference of an analog image signal output according to a quantity of light at a time of receiving reflected light from a document, and the line period is a read period of one line in the sub scanning direction.