A function addition apparatus allows an added function to be started-up immediately after a computer has been powered on or reset. A function expansion program stored in a program storage section is called, before a start-up program starts-up an operating system, by the start-up program and is executed by the CPU. The functional expansion program controls a functional section. Under a first condition, the CPU is prevented from returning to execution of the start-up program after the function expansion program has been completed by the CPU. Under a second condition, the CPU is returned to execution of a start-up program after the function expansion program has been completed by the CPU so that the operating system is started-up. When the operating system is started-up after the CPU has returned to execution of the start-up program, the operating system or the like controls the functional section.