Delay in surface voltage decay is known in phthalocyanine binder type photoreceptor. Because of the delay, sharp electrostatic image can be formed on photoreceptor. So, the photoreceptor is called “digital photoreceptor”. The surface voltage decay mechanism is analyzed on the base of structural trap model. The model explains that the carrier macroscopic mobility increases very abruptly by the increase of free carrier. The model calculation of surface decay curve is carried out and it is found that the decay curve agrees with the experimental tendency.