a white led 53 and a charge storage sensor 54 a with on chip filter of three r g and b colors light is emitted by the white led 53 obliquely at 45 u00b0 to the transfer material 11 having fixed patches 71 to 73 and the intensity of light diffusedly reflected at 0 u00b0 is detected by the charge storage sensor 54 a with the rgb on chip filter the light receiving portion of the charge storage sensor 54 a with the rgb on chip filter is a sensor 54 b that has independent r g and b pixels the charge storage sensor 54 a with the rgb on chip filter may be a photodiode or several sets of three r g and b pixels may be arranged side by side the incident angle may be 0 u00b0 and the reflection angle may be 45 u00b0 the charge storage sensor may be made up of an led which emits beams of three r g and b colors or more and a sensor with no filter next a method of correcting the density or chromaticity variations in the scan direction according to the first embodiment will be described in the first embodiment a plurality of color sensors 42 are arranged over a whole image forming region in the scan direction as shown in fig5 in this embodiment three color sensors 42 1 to 42 3 are arranged however the number of color sensors 42 need.