The present invention provides an optical transmitter that enables to distinguish failure modes of the auto-power control (APC) of the laser diode (LD). The transmitter provides the APC circuit comprised of the driver for supplying the driving current, the LD, the PD for monitoring a portion of the output light from the LD, and the control unit for setting the driving current. The control unit monitors, at regular intervals, the bias current to the LD and the time differential of the bias current. The control unit only stops the APC circuit when both the bias current and the time differential of the bias current exceed respective references.