Detailed configuration of the image reader 24 will be described below with reference to FIGS. 2 and 3. As shown in FIG. 2, the image reader 24 includes a light source 30, a light receiver 31, and a rod lens array 32. The light source 30 includes light emitting diodes to emit light in three colors, which are red, green, and blue. Light emitted from the light source 30 may reflect on the object surface of the original sheet GS facing the image reader 24, and the rod lens array 32 may focus the reflected light on the light receiver 31. The light emitting diodes may be switched to emit light in the three colors consecutively to read a line of image extending in the main-scanning direction MD on the original sheet GS. The light source 30 includes a chip of light emitting diodes in the three colors and a light guide to guide the light from the light emitting diodes in the main-scanning direction MD.

A white reference board 34 is disposed at a position to face the image reader 24 across the conveyer path 20. The white reference board 34 has a same reflection rate as white, which is a base color of the original sheet GS. When no original sheet GS is in the conveyer path 20 between the white reference board 34 and the image reader 24, the light from the light source 30 may reflect on the white reference board 34, and the reflected light may be received by the light receiver 31 through the rod lens array 32.