Various functions can be easily added using print data output from an existing application without modifying the existing application program. A function expansion method for executing a process that expands the functions of an application 201 runs on a computer 2 that operates according to an operating system on the operating system kernel layer 230, executes a specific process according to an application 201 on an application layer 220, and has one or more communication ports 209A and/or 209B, includes a step of acquiring print data output from the application 201 on the operating system kernel layer 230, and a step of performing at the application layer 220 a process expanding the function of the acquired print data, and outputs the print data after processing to a suitable communication port 209A and/or 209B.