To enable a sheet end to be detected accurately and certainly in any state.

The sheet end of a widthwise direction and a lengthwise direction of the sheet to be conveyed is detected by the steps of converting R, G and B signals read by a color CCD sensor 6 into data of digital bit by A/D converters 11r, 11g and 11b, further converting into data a*, b* of lightness data L* in a CIE uniform color space and the data a*, b* of chromaticness indices showing a hue by a 3-color conversion matrix unit 12, converting the data a*, b* into saturation data C* by an a*, b* to C* conversion table 13, inputting together with the lightness data L* to a sheet end detector 14, then synchronizing at least one change of the lightness data L* and the saturation data C* with a main scanning synchronizing signal FS clock and a sub-scanning synchronizing signal SS clock, and detecting it.