To provide a sheet-fed printer which can perform sure two-sheet detection by evaluating whether a threshold value and a sensor have elements of incorrect judgment.

The sheet-fed printer 1 having a paper feed device 3 for partially stacking and continuously conveying sheets 9, aligning the front ends of the sheets 9 by means of a front gauge 45 and transferring the sheets to a post processing section, includes: a two-sheet detection sensor 49 which is installed in an upstream side position of a front gauge 45, and has a wave transmission sensor 57 transmitting ultrasonic waves and a wave reception sensor 59 receiving ultrasonic waves, which are provided oppositely above and below across the sheets 9; a two-sheet detection timing sensor 61 which sets the detection timing of the detection sensor 49; and a missfeeding paper sensing means 50 which judges the presence or absence of the two-sheet paper feeding using the detection result of the two-sheet detection sensor. The missfeeding paper sensing means 50 is so configured as to output a warning when the minimum detection value of the two-sheet detection sensor 49 becomes greater than the prescribed range of the threshold value wherein the minimum detection value of the two-sheet detection timing sensor 61 is judged as the two-sheet detection in a period when the two-sheet detection timing sensor 61 is set to non-detection.