To provide a detector for incident light having a long wavelength that uses a low-priced, high-sensitivity photosensitive device which contains silicon-based germanium, and to provide its detection method.

The photodetector (100) has a first phototransistor (102) which includes germanium, and is structured such that it generates an output current signal in response to incident light, a second phototransistor (104), that is electrically linked to the first phototransistor to generate a reference current signal, and an opaque layer that covers the second phototransistor to shield incident light so as to make the reference signal independently of the incident light. The first and second phototransistors (104) have a silicon-germanium light-absorbing region, and incident light is detected from the output current and the reference current.