A color image reading apparatus is provided wherein a color picture image is read out by a monolithic three line sensor (4) through a reflection type one-dimensional blazed diffraction grating (3) acting as a color separating means. The diffraction grating is concavely curved toward the line sensor, and a light beam, diverging after being focused by an image forming optical element having a predetermined refracting power only in a sub-scan cross section, is color-separated into three color light components by the diffraction grating and re-imaged on the surface of the three line sensors (8,9,10) located at a predetermined position. As a result, the blur of ± 1 order diffracted lights in a sub-scanning direction can be prevented which is due to the deviation of a reflective diffraction angle resulting from the difference in an incident angle of respective lights on the grating surface in the sub-scan cross section. At this time, a convergent spherical wave that is a light beam emerging from the image forming optical system enters the diffraction grating.