A program writing method in which a program is written into a flash ROM that a microcomputer includes therein includes: a generating step for generating a version representative value indicating a version of a source directory from predetermined types of files included in the source directory; an additionally writing step for additionally writing the version representative value into a source file included in the source directory; and a program writing step for writing a program corresponding to the source directory generated by compiling the source file into which the version representative value has been additionally written into the flash ROM.