The bulk-ionized photoconductivity of C60 is reported as an origin of the bias-dependent linear change of the photocurrent in copper phthalocyanine (CuPc)/C60 planar heterojunction solar cells, based on the observation of the variation of the bias-dependent photocurrent on excitation wavelengths and the thickness-dependent photocurrent of the C60 layer. A theoretical model, which is a combination of the Braun-Onsager model for the dissociation of excitons at the donor/acceptor interface and the Onsager model for the bulk ionization of excitons in the C60 layer, describes the bias-dependent photocurrent in the devices very well. The bulk-ionized photoconductivity of C60 must generally contribute to the photocurrent in organic photovoltaics, since fullerene and fullerene derivatives are widely used in these devices.