PURPOSE: To detect jamming of a stacker by locating a jamming sensor in a position spaced a certain distance from the upper surface of stacked blanks irrespective of the size of folded blank.

CONSTITUTION: A detector 402 detects detecting beams irradiated from a detecting beam generator 401. A distance l between the upper surface end A of stacked blank 112 and the vertical is held and the positional relationship between a guide plate 115 and the vertical is held such that the distance l maintains always a same value even when the folding size of a blank 101 is varied. Hence jamming of a stacker can be detected.