The optical semiconductor device includes a spot-size converter formed on a semiconductor substrate. The spot-size converter has a multilayer structure including a light transition region. The multilayer structure includes a lower core layer, and an upper core layer having a refractive index higher than that of the lower core layer. The width of the upper core layer is gradually decreased and the width of the lower core layer is gradually increased in the light transition region. Both sides and an upper side of the multilayer structure are buried by a semi-insulating semiconductor layer in the light transition region. Light incident from one end section of the spot-size converter is propagated to the upper core layer. The light transits from the upper core layer to the lower core layer in the light transition region, is propagated to the lower core layer, and exits from the other end section thereof.