A control unit calculates target power to be generated based upon a bypass engaging voltage used as a reference voltage at which a capacity adjustment circuit executes a capacity adjustment, and issues a power generation command for an inverter so as to generate the target power if it is decided that the capacities of the individual cells need to be adjusted. Pressure is monitored in an insulating liquid of a tap changer for a period of time in which switching of the tap changer is carried through. The preliminarily molded porous member is inserted into a cavity of a product mold.