Abstract: A circuit for driving a semiconductor optical modulator which has a first semiconductor layer having a first conduction type, a second semiconductor layer having a second conduction type, and a modulation active layer sandwiched with the first and second semiconductor layers, wherein the semiconductor optical modulator modulates light emission from a semiconductor laser diode having at least a third semiconductor layer having the first conductivity type, the third semiconductor layer which is electrically connected to the first semiconductor layer of the semiconductor optical modulator, the driving circuit is provided with an output circuit for outputting a drive signal to the semiconductor optical modulator based on a signal for optical modulation inputted from the outside, and a compensation circuit for regulating a resistance of bypass channel of current passing in the semiconductor optical modulator by a drive signal of the output circuit based on the signal for optical modulation and compensating a volta