A Mach-Zehnder type semiconductor device comprises electrodes for injecting carriers into first and second branch waveguides so as to change reflection indexes of the first and second branch waveguides. The device further includes electrodes which are placed above either one of the first or second branch waveguides or both of the first or second branch waveguides so as to remove the carriers.The Mach-Zehnder type semiconductor device can adjust a phase difference between first and second split lights transmitting through first and second branch waveguides, respectively, even if the lengths of the first and second branch waveguides are deviated from those designed values. The Mach-Zehnder type semiconductor device also can suppress optical loss of the first and second lights and generate an outgoing light without deterioration.