In order to elucidate the photocurrent multiplication mechanism observed in the photoconducting organic pigment films, the electric field distribution formed by the space charges accumulated near organic/metal interface having imperfect contact and the flow of photogenerated carriers were numerically simulated. It was found that high density of surface charges, namely, charge accumulation appears in the steady state when the photogenerated carriers are supplied continuously, and that the accumulated charges staying at the non-contact organic film interface can provide enough high field for tunneling charge injection leading to multiplication process. These results indicate the validity of our previously proposed “structural trap model” that the charge accumulation occurs at blind alleys lying on the rough film surface of the organic/metal interface. Indeed, by introducing lower surface mobility based on the concept of rough surface of the orgainc film, the typically observed multiplied photocurrent response was well reproduced.