To enable a calculation expression to be registered with a label printer or to be changed and to cause the label printer to execute calculation or print a calculation result simply by sending data to be used in calculation.
A CPU11 registers a calculation expression which is specified by a calculation expression registration command received via an external interface 17 with an EEPROM21. When data to be used in calculation is sent via the external interface 17, the CPU11 uses the data and the registered expression for calculation, sends the calculation result to a printer controller 15, and, at the same time, instructs the printer controller to print. The print controller 15 controls a print head 5 based on the command from the CPU11 and prints the calculation result on a label.