A microcomputer includes a universal serial bus (USB) interface circuit wherein a program is written from a host to the microcomputer at high speed by utilizing a USB cable for connecting the host to a peripheral device. Nonvolatile memory, such as flash ROM, for program storage has a first program area to which program data transmitted from a personal computer is to be written and a second program area storing a write control program. Program data stored temporarily in RAM is written to the first program area of the flash ROM through a USB interface circuit in accordance with the write control program.