An image formation apparatus stably controls a drive current of a laser diode irrespective of change in density. In the apparatus, a current restriction circuit restricts a current ILD according to the output light quantity. If the current ILD increases and thus approaches to the value of a reference current IPO, the circuit outputs the current restricted to be the certain value substantially equal to the current IPO. Then, current buffers and, a current subtraction circuit, a switching circuit and a current addition circuit cooperatively supply the function current obtained from the input and output currents of the circuit to the cathode of the laser diode.