To reduce cost of a paper feeder.

When a main detecting member 61, for detecting a fed paper sheet, and an auxiliary detecting member 62 are rotated by a fed paper sheet by the main detecting member 61, the auxiliary detecting member 62 too is rotated temporarily. An optical sensor 65 is provided only on the auxiliary detecting member 62. The single optical sensor 65 maintains an ON condition when the paper sheet is fed normally, and becomes the ON condtition temporarily when the paper sheet is skewed. A control circuit 36 judges whether the paper sheet is fed normally, or, is skewed, based on information from the optical sensor 65.