A waveguide includes a core and a cladding. The core has an inlet on which light is incident. The core includes a front portion and a rear portion located between the front portion and the inlet. The front portion and the rear portion each have a thickness that is a dimension in a first direction and a width that is a dimension in a second direction. The first direction is orthogonal to a propagation direction of the light. The second direction is orthogonal to the propagation direction of the light and the first direction. The thickness of the front portion decreases with increasing distance from the inlet.