PURPOSE: To certainly perform change-over operation, by mounting not only a function for automatically changing over a plurality of data inputted to a printer and holding a definite time but also a function capable of performing printing without deforming a printing format.

CONSTITUTION: Faster data among data sent from a display apparatus 1 or a calculator 2 is detected by a change-over mechanism 21 to be printed by a printer apparatus 22. At this time, a busy signal is sent to machinery of slower data and the reception of data transmission is temporarily stopped. When printing is finished in the printer apparatus 22, the busy signal is released after a definite time in order to make it possible to again receive data from the display apparatus 1 or the calculator 2. By providing a definite time when data is changed over by the change-over mechanism 21, data is introduced from the calculator 2 by timing and the disorder of a slip format can be prevented.