A semiconductor storage device includes a program ROM and a flash memory, and the program ROM includes a program data storing area and a dummy data storing area, and the flash memory includes a program data storing area and a dummy address storing area. An address comparison circuit compares an input address and a dummy address being stored in the dummy address storing area with each other. An output inhibiting circuit allows to read program data from the program ROM when the both addresses are not coincident with each other, and inhibits the program data from being read at a time of incoincidence. In a case of the incoincidence, an address conversion circuit produces a read address of the flash memory, whereby the program data is read from the flash memory.