A ROM cartridge 20 and a disk drive 30 are detachably attached to slots 103 and 104 of an information processing unit 10. A magnetic disk 40 is detachably attached to a slot 302 of the disk drive 30. A semiconductor storage device storing a game program and data is provided inside of the ROM cartridge 20. The information processing unit 10 detects whether the ROM cartridge 20 is attached to the slot 103 and whether the disk drive 30 is attached to the slot 104. Then the information processing unit 10 performs predetermined information processing on a basis of program data stored in the ROM cartridge 20 or the disk drive 30 when detecting that the ROM cartridge 20 or the disk drive 30 is attached to the slot 103 or 104, and starts information processing on the basis of the program data stored in the ROM cartridge 20 when detecting that both of the ROM cartridge 20 and the disk drive 30 are attached to the slots 103 and 104.