Both high light receiving sensitivity and high speed of a photodiode are achieved at the same time. The photodiode is provided with a semiconductor layer (1) and a pair of metal electrodes (2) which are arranged on the surface of the semiconductor layer (1) at an interval (d) and form an MSM junction. The interval (d) satisfies the relationship of λ>d>λ/100, where λ is the wavelength of incident light. The metal electrodes (2) can induce surface plasmon. At least one of the electrodes forms a Schottky junction with the semiconductor layer (1), and a low end portion is embedded in the semiconductor layer (1) to a position at a depth less than λ/2n, where n is the refractive index of the semiconductor layer (1).