PURPOSE: To reduce the operation stopping time of a printer due to lack of printing sheet by jumping a weight of spherical shape placed on a folded printing sheet with respect to the remaining unprinted sheet and operating a sensor through a rockable lever in response to the jumping height.

CONSTITUTION: A weight 4 of spherical shape is hung via a hanging string 5 which is adjusted in length along a guide plate 2 from the end of a rockable arm which has a fulcrum 61 supported to a housing frame 1 on sheets 10 in an unprinted sheet stacker which has a front guide plate 2 and a rear guide plate 3 for guiding the folds of a folded printing sheet 10. An arms 6 is lifted by a balance spring 7 from the frame 1, the sheet 10 in the stacker is exhausted in a direction of broken line with an arrow, is pulled and lowered by a weight slightly heavier than the strength of the spring 7 upon reduction of the thickness of the sheet 10. A sensor 8 supported by the frame 1 is retained by the arm 6, thereby operating the sensor 8.