Valve, abnormality diagnosis method of valve

To enable accurate and simple diagnosis of an operation abnormality of a valve.A valve V1 capable of detecting an operation abnormality has a magnet M1 that is attached in the vicinity of a pressing adapter 52 of a stem 53 that slides according to an opening/closing operation of the valve V1, and a magnetic sensor M2 that is attached to a surface acing the stem 53, inside the pressing adapter 52 that presses a peripheral edge of a diaphragm 51, and detects a change in a distance between the magnet M1 and the magnetic sensor M2. Further, an abnormality determination mechanism compares the change in the distance between the magnet M1 and the magnetic sensor M2 at the time of abnormality diagnosis detected by the magnetic sensor M2 and a previously measured change in the distance between the magnet M and the magnetic sensor M2 at the time of normality, and determines whether or not there is an abnormality.