Disclosed is a planar waveguide including: a core (11) which is a flat plate through which light propagates; a cladding (12) which is a flat plate for reflecting the light in a state of being joined to an upper surface of the core (11); and a cladding (13) which is a flat plate for reflecting the light in a state of being joined to a lower surface of the core (11), in which each of the claddings (12) and (13) is a multilayer film in which multiple films made from different materials are layered. As a result, a material having a low index of refraction can be used as the material of the core (11), and the limit on materials usable as the material of the core (11) is relaxed.