To obtain an image with true resolution by adopting a variable resolution system moving an optical system in matching with the size of an original.

Suppose that a moving position A1 is a 1-inch read range X and a moving position A2 is an 8-inch read range X, then a position shown in broken lines represents the moving position A1 of 1-inch read optical unit 7 and the position shown in one-dash chain lines represents a moving position A2 of an 8-inch read optical unit 7, and when number of picture elements of a line CCD sensor 72 is constant to be 4800-dot for example, a resolution Y of 4800dpi at the moving position A1 for the read range X to be 1-inch is obtained and a resolution Y of 600dpi at the moving position A2 for the read range X to be 8-inch is obtained.