PURPOSE: To enable to accurately detect registering of a printing paper, with a patch code, etc. unnecessitated, by discriminating whether a register mark of the paper is off the register reference position, through arithmetic operation to set the range of tolerance and register reference position according to a relative positional signal of register mark and an absolute positional signal corresponding to the running angle of a rotational drum.

CONSTITUTION: A light source lamp 41 illuminates register marks 3a and 3b, and the light reflected therefrom is received by a photoelectric conversion element 42 which subsequently sends forth an output signal to a register mark judging circuit 6, wherein a pulse generator 5 outputs pulses which are synchronized with a rotational drum 1, and said pulses are measured by a instrumentation circuit 7. The value measured therein represents the rotational angle of the rotational drum 1, and becomes as a means to output absolute positional signal of the register marks 3a, 3b. The absolute positional signals thus obtained and the relative positional signals obtained from the judging circuit 6 are stored in a memory circuit 8. These signals are read, and a tolerance for bad register and a reference position are calculated by an arithmetic operation circuit 9, and the calculated result is outputted from a register position judging circuit 10 as a judging signal E.