A Mach-Zehnder modulator includes: a first arm waveguide having first to third waveguide portions, the third waveguide portion being curved to couple the first and second waveguide portions with each other; a second arm waveguide having first to third waveguide portions, the third waveguide portion being curved to couple the first and second waveguide portions with each other, and a differential signal conductor having first and second signal conductors for driving the first and second arm waveguides, respectively. The first signal conductor has a first conductor portion and a first intersecting conductor portion connected thereto. The second signal conductor has a first conductor portion and a second intersecting conductor portion connected thereto. One of the first and second intersecting conductor portions includes an upper conducting layer, and the other includes a lower conducting layer. The upper conducting layer extends on the lower conducting layer apart therefrom.