Image reader

An image reader comprises a monolithic 3-line sensor having three one-dimension sensor arrays such as solid-state imaging devices arranged on one substrate with each array being spaced by a predetermined distance along a direction perpendicular to the array direction, a focusing optical system for focusing an image of an object onto the sensor, and a one-dimension blazed diffraction grating arranged in a light path between the focusing optical system and the sensor, closer to the sensor than to an exit pupil of the focusing optical system for spectroscopically separating a light from one point on the object by a diffraction effect to color-decompose it into three colors perpendicularly to the array and directing the color-decomposed lights to the corresponding sensor arrays. Distances from a center sensor array of the 3-line sensor to two other sensor lines are different from each other.