According to an embodiment, a semiconductor device includes a primary side lead, a light-emitting element electrically connected to the primary side lead, and a thyristor-type light-receiving element. The light-receiving element includes a first face for detecting light emitted from the light-emitting element, and a second face provided on an opposite side of the first face. The light-receiving element includes an anode electrode, a cathode electrode, and a gate electrode that are provided on the first face. The device further includes a secondary side first lead electrically connected to the anode electrode, a secondary side second lead electrically connected to the cathode electrode, and a secondary side third lead electrically connected to the gate electrode. The secondary side third lead is connected to the second face of the light-receiving element.