In an image forming apparatus, a paper sheet cassette stores a bunch of paper sheets to be fed. A media sensor unit measures a thickness and a surface roughness of one paper sheet fed and transported from the paper sheet cassette. A residual sheet amount sensor detects a residual amount of the paper sheets in the paper sheet cassette. A residual sheet number estimating unit estimates the number of residual paper sheets in the paper sheet cassette on the basis of (a) a thickness of a bunch of the paper sheet in the paper sheet cassette, the thickness of the bunch corresponding to the residual amount detected by the residual sheet amount sensor and (b) the thickness of the one sheet and the surface roughness of the one sheet measured by the media sensor unit.