This optical device includes at least one magnetic element including a first ferromagnetic layer, a second ferromagnetic layer, and a spacer layer sandwiched between the first ferromagnetic layer and the second ferromagnetic layer, a laser diode, and a waveguide, in which the waveguide includes at least one input waveguide optically connected to the laser diode and an output waveguide connected to the input waveguide, and at least some of light propagating in at least one of the input waveguide and the output waveguide is applied to the magnetic element.