A butterfly valve includes a purge gas flow path supplying a purge gas for preventing inflow of a control fluid into a magnetic fluid sealing portion from a flow path through an insertion hole. The purge gas flow path is provided with an opening and closing valve for opening and closing the purge gas flow path and a pressure gauge for measuring a pressure value in the purge gas flow path, in the stated order from the upstream side. The purge gas flow path is connected to a portion between the magnetic fluid sealing portion and the flow path of the butterfly valve on the downstream side of the pressure gauge, and communicates with the flow path through the insertion hole. The butterfly valve further includes a control device that controls at least the opening and closing valve. The control device is provided with a monitoring program for monitoring a pressure value.