There is provided a sheet feeder for a sheet-fed press which allows an optimum separation state of printing sheets 27 to be established readily and certainly. Air is jetted out from an injection nozzle 6 toward the upper part of a bundle 2 of printing sheets to thereby float up printing sheets 27. A top printing sheet 27 thus floated is absorbed by an absorption foot 8 and conveyed to a printing process. The number of floating printing sheets 27 is detected using photoelectric sensors 21 and 22, and in order to establish the optimum separation state, is adjusted by varying the injection air quantity from the injection nozzle 6 or by moving the paper pressure bar 4 in directions of arrows 93 and 94. Further, by equalizing outputs G1 and G2 from detection areas M1 and M2, it is possible to place the top printing sheet 27 in parallel with the absorption surface 8Q of the absorption foot 8 and realize secure absorption. A fuzzy inference system may be used for adjustment.