In a portable computer having an extension bus connector connected to a system bus, and a memory connected to an internal bus, and requiring a refresh operation, whether or not an extension unit is connected to the extension bus connector is checked. When the extension unit is connected to the extension bus connector, a refresh signal is output onto the system bus. When the extension unit is not connected to the extension bus connector, the refresh signal is output onto the internal bus. Only when the extension unit is connected to the extension bus connector, the refresh signal is output onto the system bus. When the refresh cycle of a memory incorporated in the portable computer is longer than that of a memory connected to the extension unit, the presence/absence of connection of the extension unit is detected, and a refresh mode is set to be a normal refresh mode.