Quantum wire is formed on the bottom of a V-shaped groove in a V-grooved substrate as a channel between source and drain electrodes or as at least part of the channel. A photocarrier accumulation region is provided within the quantum wire or at a position connected to or adjacent to the quantum wire for accumulating charges generated when light shines onto a photosensitive region that comprises at least a clad layer that covers the quantum wire. A recess is provided in the upper clad layer to localize the photocarrier accumulation region. As a result, it is possible to provide a photodetector that exhibits high sensitivity, high speed and low power consumption in an expanded wavelength region. It is also possible to provide a photodetector capable of constructing core portions thereof by one-time selective growth.