According to this printer, effective extension conditions for extending extension memories to plural memory-installing slots are preliminarily stored in a ROM table. The presence or absence of the extension memories extended to the plural memory-installing slots and the capacity thereof are identified. On the basis of the identification result, it is determined whether the installing state of the extension memories is correct. If the installing state is not correct, a correct installing state is determined from the ROM table, and it is printed by a printing mechanism. Accordingly, the incorrect installing state of the extension memories is detected, and the information on the correct installing state is printed.