Band engineering using the van der Waals heterostructure of two-dimensional materials allows for the realization of high-performance optoelectronic devices by providing an ultrathin and uniform PN junction with sharp band edges. In this study, a highly sensitive photodetector based on the van der Waals heterostructure of WSe2 and MoS2 was developed. The MoS2 was utilized as the channel for a phototransistor, whereas the WSe2-MoS2 PN junction in the out-of-plane orientation was utilized as a charge transfer layer. The vertical built-in electric field in the PN junction separated the photogenerated carriers, thus leading to a high photoconductive gain of 10(6). The proposed phototransistor exhibited an excellent performance, namely, a high photoresponsivity of 2700 A/W, specific detectivity of 5 x 10(11) Jones, and response time of 17 ms. The proposed scheme in conjunction with the large-area synthesis technology of two-dimensional materials contributes significantly to practical photodetector applications.