Various functions can be easily added using print data output from an exist lag application without modifying the existing application program. A function expansion method for executing a process that expands the functions of an application runs on a computer that operates according to an operating system on the operating system kernel layer, executes a specific process according to an application on an application layer, and has one or more communication ports, includes a step of acquiring print data output from the application on the operating system kernel layer, and a step of performing at the application layer a process expanding the function of the acquired print data, and outputs the print data after processing to a suitable communication port.