Abstract: A diaphragm member is formed in a spherical cap shape that is centered on an extension line of a central axis of an operation rod and bulges toward the operation rod. The diaphragm member is held, at its vertex, from both sides in a direction parallel to the central axis, between a first holding piece contacting the diaphragm member from the operation rod side and a second holding piece contacting the diaphragm member from the valve element side. The first and second holding pieces each has a circular shape positioned coaxially with the operation rod. The diameter of the first holding piece is larger than the diameter of the second holding piece. A surface of the first holding piece facing the diaphragm member is a convex spherical surface bulging toward the diaphragm member and having the center on the central axis of the operation rod.

Abstract: The present invention has: a valve body; a servo motor having a drive shaft for operating the valve body; a position control mode for controlling the drive shaft to move the drive shaft to a target position, and a thrust control mode for controlling the drive shaft to cause the drive shaft to obtain a target thrust; and a control program that, when operating the valve body from a valve open position to a valve close position, controls the servo motor in the position control mode from the valve open position until the valve body reaches a predetermined position before reaching the valve close position, and, from the predetermined position, switches the control of the servo motor from the position control mode to the thrust control mode with a target thrust of zero, and causes the valve body to reach the valve close position only by the biasing force.