A Mach-Zehnder modulator comprises first to third semiconductor structures provided on first to third areas of a primary surface of a conductive semiconductor region, respectively. The second semiconductor structure includes a semiconductor laminate and a first contact portion thereon. The third semiconductor structure includes the semiconductor laminate and a second contact portion thereon. The first and second contact portions constitute a contact layer. The second semiconductor structure has first and second waveguide sides. The first contact portion has an edge which terminates the contact layer and extends in a direction of a first reference plane on the top of the semiconductor laminate to reach the first waveguide side. The first reference plane and a top of the second semiconductor structure intersect with each other to define a line of intersection obliquely-crossed with an upper edge of the first waveguide side at a first angle different from a right angle.