An object of the present invention is to provide a command data conversion device and printing apparatus that can correctly interpret print command data even where the codes or the parameter codes assigned to the commands in the print command data change, or where the data output sequence changes. The plunger and syringe barrel have cooperating locking elements so that the plunger is locked after use within the syringe barrel. A controller of the fuel supply system generates commands for closing the valve to the high-pressure pump during starting of the engine before the cylinders are discriminated from each other, such that a period of the generated commands is shorter than a half of the cycle of operation of the high-pressure pump during starting of the engine.