Firmware stored in a printer such as an inkjet printer can be rewritten by executing a firmware rewriting process which is not dependent upon the internal processing status of the printer. A printer for executing a printing process based on print data sent from a host computer has a rewriting unit for executing a firmware rewriting process, a print unit for executing processes other than the firmware rewriting process, a rewrite command interpreting unit for detecting a rewrite command that precedes the transmission of rewrite data required for the firmware rewriting process, and a control unit for executing the firmware rewriting process with a priority over all other processes when a rewrite command is detected.