A transmission type photocathode includes a light absorption layer 1 formed of diamond or a material containing diamond as a main component, a supporting frame 21 for reinforcing the mechanical strength of the light absorption layer 1, a first electrode 31 provided at the plane of incidence of the light absorption layer 1, and a second electrode 32 provided at the plane of emission of the light absorption layer 1. A voltage is applied between the plane of incidence and plane of emission of the light absorption layer 1 to form an electric field in the light absorption layer 1. When light to be detected is made incident and photoelectrons occur in the light absorption layer 1, the photoelectrons are accelerated to the plane of emission by the electric field formed in the light absorption layer 1, and emitted to the outside of the transmission type photocathode.