A method of manufacturing an active pixel sensor having a plurality of pixels, each of the pixels having a photodiode formed by a part of a first semiconductor region of a first conductive type and a second semiconductor region of a second conductive type, and a transfer transistor for transferring a charge carrier from the photodiode, includes the steps of preparing a substrate on which the first semiconductor region of the first conductive type is formed, forming a mask to form the second semiconductor region on the substrate, forming the second semiconductor region using the mask, and forming a gate of the transferring transistor after forming the second semiconductor region. The gate of the transferring transistor overlaps the second semiconductor region in a planar view.