Abstract: An optical waveguide type photodiode has a plurality of semiconductor layers formed one upon another on a semiconductor substrate and including an optical absorption layer sandwiched between a pair of optical confinement layers for guiding incident light in parallel with the semiconductor layers, wherein a light absorption quantity per unit length of an optical waveguide area constituted by the optical absorption layer is substantially constant throughout the entire area thereof. Specifically, the optical confinement factor .GAMMA.(x) of the optical waveguide area is set so as to increase with guided distance x of light. Preferably, a device structure is employed in which the thickness d(x) of the optical absorption layer increases with the guided distance x of light.