PURPOSE: To perform a reliable and easy management of counting of printed sheets, by a method wherein a numbering device for printing a manegement number is mounted to a sheet printer, main sheets and thickness adjusting sheets are automatically discriminated, and numbers of the numbering device are transferred to only the main sheets.

CONSTITUTION: Flip-flop circuits FF1 and FF2 hold information depending on whether a sheet is a main sheet or a thickness adjusting sheet corresponding to detecting outputs of a color detector CLD and a drum-in detector DID. Corresponding to a synchronizing pulse from a pulse generator PG, if it is the main sheet, an output of an AND gate G1 is brought to "H", flip-flop circuit FF1 is set, and an output Q is brought into "H". An output of the flip-flop circuit FF1 is applied also to a shift registor SR1, and a shifting is made by means of a synchronizing pulse. Corresponding to the output of the shift registor SR1, a driver DR produces a drive output to drive a cam for converting a number of a numbering device mounted to a number drum.