A waveguide PIN photodiode is provided. The waveguide PIN photodiode includes a lower light guide layer, a light absorption layer, an upper light guide layer, and a cladding layer. The lower light guide is formed on a substrate, and the light absorption layer is formed on the lower light guide layer. The upper light guide layer is formed on the light absorption layer, and the cladding layer is formed on the upper light guide layer. The lower light guide layer, the light absorption layer, and the upper light guide layer constitute a core layer, which is an optical waveguide, and graded index distribution is symmetrically formed in a depth direction, centering around the light absorption layer having a highest refractive index.