a motor;

a flow path in which a control fluid flows;

a rod having one end joined to the motor and the other end inserted in the flow path; and

a butterfly valve element joined to the rod in the flow path, wherein

the butterfly valve comprises a magnetic fluid sealing portion between the motor and the flow path,

the magnetic fluid sealing portion is configured to prevent leakage of the control fluid from the flow path to the motor through an inserted portion of the flow path in which the rod is inserted,

the butterfly valve comprises a purge gas flow path to supply purge gas to the butterfly valve, the purge gas preventing inflow of the control fluid to the magnetic fluid sealing portion through the inserted portion from the flow path,

the purge gas flow path is provided with an opening and closing valve to open and close the purge gas flow path and a pressure gauge to measure a pressure value of the purge gas flow path in this order from an upstream side,

the purge gas flow path is connected between the magnetic fluid sealing portion and the flow path of the butterfly valve and communicated with the flow path through the inserted portion on a downstream side of the pressure gauge,

the butterfly valve comprises a control device to control at least the opening and closing valve, and

the control device includes a monitoring program to monitor the pressure value.