A method for manufacturing an image sensor includes forming circuitry including a metal line over a semiconductor substrate, forming a photodiode over the metal line, and forming a contact plug in the photodiode such that the contact plug is connected to the metal line. The forming of the contact plug includes performing a first etch process to etch a portion of the photodiode, and performing a second etch process to expose a portion of the metal line by using a byproduct generated in etching, to form a via hole for the contact plug in the photodiode.