The present invention provides a polymer optical waveguide having a core, an under cladding and an over cladding, in which: the polymer optical waveguide has a first section on one end side in the light propagation direction where no portion of the over cladding exists and the core and the under cladding are exposed, and a second section on the other end side in the light propagation direction where the core is covered with the under cladding and the over cladding; and a relative refractive index difference among the core, the under cladding and the over cladding, a core width and core height in an end surface at the one end side of the first section, and a core width and core height in an end surface at the other end side of the second section satisfy predetermined relationships.