Managing commands in a buffer is simplified while continuing to enable immediately executing real-time commands. A control unit of a printer sequentially writes commands received from a host computer to a first buffer and sequentially reads the commands. If the read command is a real-time command, the control unit executes the command. If the read command is a normal command, the control unit writes the command to a second buffer without executing the command from the first buffer. The control unit sequentially reads and executes normal commands written to the second buffer.