A spot size converter has a first core, a larger second core, and a clad disposed on a substrate. The first core has a rectilinear cross-sectional shape and is embedded in the clad, except at its ends. One of these ends has a sloping surface along which the thickness of the first core tapers gradually to zero. The second core, which has a refractive index intermediate between the refractive indexes of the first core and clad, sits on the clad and covers the sloping end surface of the first core. Light propagates through the first core, then through the second core into an external optical device, or propagates from an external optical device through the second core into the first core. This arrangement provides a spot size converter having an easily manufacturable structure and no polarization dependency.