The depth of field is extended by using a simple structure. Provided is an imaging device comprising: an objective optical system that comprises an aperture stop that is disposed at an intermediate position on an optical axis and that has an opening that allows incident light from an object to pass therethrough; and an imaging element that acquires an optical image of the object, which is formed by the objective optical system, wherein pixels of the imaging element are arranged in a square along two mutually orthogonal axial directions, and the aperture stop includes a light-blocking portion at a portion aligned with the optical axis, the light-blocking portion having a square shape with sides inclined at 45° relative to the directions in which the pixels are arranged.