In an illuminance sensor, a slow axis of a first quarter-wave plate has a relation of +45° or −45° in regard to a polarization direction of a first linear polarization plate; a relation of a slow axis of a first portion of a second quarter-wave plate in regard to a polarization direction of a second linear polarization plate is the same with relation of the slow axis of the first quarter-wave plate in regard to the polarization direction of the first linear polarization plate, that is, +45° or −45°; and a relation of a slow axis of a second portion of the second quarter plate in regard to the polarization direction of the second linear polarization plate is −45° or +45° that is opposite in sign to the relation of the slow axis of the first quarter-plate in regard to the polarization direction of the first linear polarization plate.