PURPOSE: To realize a compact design as a whole and to simplify an optical system part for a color image forming device, by adding a switch type color separating filter device to a short-focus lens array.

CONSTITUTION: For a switch type color separating filter device 8, three color separating filters 81B, 81G and 81R for blue B, green G and yellowish red R are attached and supported to a common support member 84 which can turn centering on the shafts 82 and 83 which are parallel to the lengthwise direction of a short-focus lens array 7 and set close to the array 7 and at the front and rear positions of the member 84. Then the member 84 is turned by a prescribed angle and with each color separating copy, and the filters 81b, 81G and 81R are put successively into an image forming optical path in a switching way.