A planar lightwave circuit is provided. The planar lightwave circuit includes a waveguide and a spotsize converter which is a part of the waveguide, wherein a core is embedded in a cladding in the waveguide, and the spotsize converter is located near an end face of a substrate on which the planar lightwave circuit is formed, the spotsize converter including: a core width fine-tuning part in an end face side of the substrate; and a core width converting part which follows the core width fine-tuning part; wherein core width of the spotsize converter is minimum at an end face of the substrate, a mean taper angle θ1 of the core width fine-tuning part is larger than 0° and smaller than a mean taper angle θ2 of the core width converting part.