Abstract: A paper stacker for an image forming apparatus is free from jams ascribable to curls of paper sheet which are apt to occur in hot and humid environments, and jams ascribable to static electricity which is apt to deposit on paper sheets in dry environments. When paper sheets are curled due to a large water content, the curl is sensed to inhibit further stacking operations. When paper sheets are electrostatically charged due to a small water content, the charge is dissipated before the paper sheets are stacked. The charge which may remain on a paper sheet having been stacked is immediately discharged when the paper sheet approaches feelers.