A planar lightwave circuit and an optical receiver which reduce degradation in signal quality is provided.A planar lightwave circuit includes: a substrate with a straight groove formed therein; a signal light input port which is formed in the substrate and receives signal light; a first planar filter part which is fitted into the groove and separates the signal light into a signal light component including a first polarization state and that including a second polarization state; a second planar filter part which is fitted into the groove and separates the local oscillator light into a local oscillator light component including the first polarization state and that including the second polarization state; a first interference part formed in the substrate; a second interference part which is formed on a side of the substrate opposite to the side of the first interference part across the groove; a first waveguide which is connected to the signal light input port and also to a reflection surface of the first planar filter part; a second and a third waveguide; a fourth waveguide which is connected to the local oscillator light input port and also to a reflection surface of the second planar filter part; and a fifth and a sixth waveguide.