PURPOSE: To easily detect the drop of a valve and valve rod.

CONSTITUTION: Regarding a back washing valve formed out of a butterfly valve, an adjusting bolt 14 capable of free advance and retreat in the axial direction of a valve rod 10 is provided on the bottom of a valve box 9 faced to the lower edge of the rod 10. Furthermore, the bolt 14 is kept in contact with the lower edge of the valve rod 10 for the support thereof at the set position. Also, a detection pin 21 capable of free advance and retreat in the axial direction of the rod 10 is laid on the bottom of the box 9 in parallel to the bolt 14 and in contact with the lower edge of the rod 10. The positional change of the rod 10 is detected via the advance and retreat amount of the pin 21.