A butterfly valve to be placed on a pipe between a vacuum chamber provided with a pressure sensor and a vacuum pump is configured to control the pressure in the vacuum chamber. The butterfly valve includes a servo motor, a butterfly valve element to be rotated about a rod connected to the servo motor in an opening or closing direction, and a control unit to adjust an opening degree of the butterfly valve element based on a pressure value of the vacuum chamber detected by the pressure sensor. The control unit is provided with an abnormal vibration detection program configured to whether or not abnormal vibration is occurring in the servo motor based on a variation amount of the opening degree of the butterfly valve element within a monitoring time while at least the pressure value of the vacuum chamber is stable.