Abstract: An amorphous silicon photosensor is disclosed, which comprises a photoconductive layer which comprises three or four amorphous silicon layers formed on a substrate, each layer containing at least one kind of atom selected from the group consisting of hydrogen, heavy hydrogen, and halogen atoms, and having two heterojunctions, in which the optical band gap of an amorphous silicon layer lying between the two heterojunctions is in the range of 1.6 to 1.8 eV, a first end amorphous silicon layer, to which light is applied, contains oxygen, has an optical band gap of 1.9 eV or more, and at least part of the layer indicates an i-type or p-type conductivity, and a second end amorphous silicon layer, to which light is not applied, contains oxygen, having an optical band gap of 1.9 eV or more, and at least part of the layer indicates an i-type or n-type conductivity.