Abstract: A storage device which consumes small amounts of power during self-refreshing. The storage device includes a memory controller which forces a dynamic RAM to start a self-refreshing operation in response to a backup instruction signal. The backup instruction signal is issued when power from a main power source is shut off. During the self-refreshing operation, RAS and CAS signal lines connected to the dynamic RAM are grounded via pull-down resistors so that these signal lines are fixed to an L-level. When electric power is supplied via the main power line, a backup power supply controller directly transfers the electric power received via the main power line to a memory circuit, whereas the backup power supply controller transfers electric power received via the backup power line to the dynamic RAM when the power supply is switched to the backup power line.