To stabilize the driving of optical modulator.

A first differential pair TR4 and TR5, second differential pair TR8 and TR9 coupled to one of transistors of the first differential pair in common, a third differential pair TR10 and TR11 coupled to the other transistor of the first differential pair, and current control circuits D2, D3, D4, and TR7 to control respective current ratios of second and third differential pairs are included to constitute a modulator driving part 42. Current ratios of second and third differential pairs are controlled to be able to adjust the output amplitude, and at this time, the change of voltages between gates and drains (Vgd) of transistors included in the first differential pair is suppressed, Thus, the driving of the optical modulator is stabilized.