A laser diode drive circuit includes: a duty control amplifier (23) that controls the duty ratio of a main signal for laser control in accordance with a duty control signal; and an AND gate (22) that outputs the duty control signal to the duty control amplifier (23), and outputs a duty control signal that controls the duty ratio of the main signal to be 0% in the duty control amplifier in accordance with a shutdown signal of a laser diode. With this structure, there is no need to input the main signal having the duty ratio controlled to a logic circuit that becomes unstable. Thus, outputs from a semiconductor laser can be shut down, and the output duty can be controlled in a stable manner.