A Mach-Zehnder type semiconductor device includes 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 light transmitted through the first and second branch waveguides, respectively, even if the lengths of the first and second branch waveguides deviate from designed values. The Mach-Zehnder type semiconductor device also can suppress optical loss of the first and second light, and generate outgoing light without deterioration.