An image density detector for an image forming apparatus, for detecting image density of an image borne by an image bearer, the image density detector comprising: a reference board mounted on a shutter of the image density sensor, the reference board having a predetermined spectral reflectance distribution closer to a spectral reflectance distribution of an image forming material than a spectral reflectance distribution of white; a light emitter configured to emit light to the reference board and the image borne by the image bearer; and a light receiver configured to receive the light emitted by the light emitter and reflected from the image and the reference board; an image density calculator configured to calculate the image density of the image based on an output of the light receiver receiving the light emitted by the light emitter and reflected from the image; and an image density detecting condition corrector configured to correct an image density detecting condition based on an output of the light receiver receiving the light emitted by the light emitter and reflected from the reference board; wherein the reference board includes a plurality of reference boards (56C; 56Y; 56M) differing from each other in spectral reflectance distribution, characterised in that the image density detector is configured to selectively use the plurality of reference boards (56C; 56Y; 56M) to correct the image density detecting condition depending on the spectral reflectance distribution of the image forming material by selecting a one of the plurality of reference boards that has a spectral reflectance distribution closer to the spectral reflectance distribution of the image forming material than the rest of the plurality of reference boards, and correcting the image density detecting condition based on the output of the light receiver receiving the light reflected from the reference board thus selected. a reference board mounted on a shutter of the image density sensor, the reference board having a predetermined spectral reflectance distribution closer to a spectral reflectance distribution of an image forming material than a spectral reflectance distribution of white; a light emitter configured to emit light to the reference board and the image borne by the image bearer; and a light receiver configured to receive the light emitted by the light emitter and reflected from the image and the reference board; an image density calculator configured to calculate the image density of the image based on an output of the light receiver receiving the light emitted by the light emitter and reflected from the image; and an image density detecting condition corrector configured to correct an image density detecting condition based on an output of the light receiver receiving the light emitted by the light emitter and reflected from the reference board; wherein the reference board includes a plurality of reference boards (56C; 56Y; 56M) differing from each other in spectral reflectance distribution, characterised in that the image density detector is configured to selectively use the plurality of reference boards (56C; 56Y; 56M) to correct the image density detecting condition depending on the spectral reflectance distribution of the image forming material by selecting a one of the plurality of reference boards that has a spectral reflectance distribution closer to the spectral reflectance distribution of the image forming material than the rest of the plurality of reference boards, and correcting the image density detecting condition based on the output of the light receiver receiving the light reflected from the reference board thus selected.