To produce a subject's image on an imager as accurately as possible, a technique that is effectively applicable to the process of assembling an optical system is provided.An imaging device includes: an imager; a plate member, to which the imager is attached; and a holding member configured to hold the plate member. The plate member has a first portion, through which at least one fixing hole has been cut, and a second portion, to which the imager is attached and through which at least one adjusting hole has been cut. The plate member is held by the holding member using a number of fastening members to be introduced through the at least one fixing hole and the at least one adjusting hole. When the first portion is fixed on the holding member, the fastening member that is introduced through the at least one adjusting hole adjusts the relative position of the second portion to the holding member.