The present invention provides an LD driver able to stably control the output power and the extinction ratio of the optical signal without regarding the coupling mode, the AC mode or the DC mode, between the laser diode and the LD driver. The LD driver includes the peak hold and the bottom hold, each detecting the peak level or the bottom level of the monitored signal, respectively. The bias driver adjusts the bias current so as to equalize the peak level of the monitored signal to the reference, REF_PEAK, while, the modulation driver adjusts the modulation current so as to equalize the bottom level of the monitored signal to the reference, REF_BOTTOM. In the present invention, the adjustment of the modulation current by the modulation driver starts with a substantial delay with respect to the stabilization of the bias current by the bias driver.