An image forming apparatus is provided with an image forming unit including an amorphous silicon photoconductor. The amorphous silicon photoconductor includes a base and a photosensitive layer provided on the base. The photosensitive layer includes a highly resistive layer, a charge-injection inhibition layer, a photoconductive layer and a surface protecting layer successively laminated on the base. The thickness of the highly resistive layer is in the range of 1 μm to 4 μm. The thickness of the photosensitive layer is in the range of 15 μm to 25 μm and the absolute value of a solid light potential of the photosensitive layer is in the range of 20 to 100 V.