A plurality of leads include a pair of differential signal leads for inputting differential signals, and a power supply lead for supplying power. A wiring pattern includes a pair of differential transmission lines connected to the pair of differential signal leads, and a power supply wiring connected to the power supply lead. A wiring board includes a first region overlapping an optical subassembly, and a second region extending from the first region so as to protrude from the optical subassembly. The pair of differential signal leads are farther away from the second region than the power supply lead. The pair of differential transmission lines are close together so as to be electromagnetically coupled to each other. The optical subassembly does not include a lead penetrating the wiring board in a region between the pair of differential transmission lines.