In a shadow RAM using a ferroelectric capacitor, a memory cell constituted by connecting a ferroelectric capacitor directly to each of storage nodes of an unloaded four-transistor SRAM cell formed of four transistors. The duct may be connected together end to end by flanged connectors formed from strip stock, that is notched at locations that correspond to the corners of the formed flange connector. A ratchet mechanism that allows the fixing member to move to an incorporated location, an attachment position, a fixed position, and a removal position in order, but stops further movement to a position where the fixing member has moved once is provided between the inner peripheral surface of the plug main body and the outer peripheral surface of the fixing member.