To accurately detect absolute cylinder displacement, in a cylinder displacement measuring device.

In order to measure cylinder displacement in a cylinder device having a cylinder 2, piston slid in the cylinder 2 and a cylinder rod 4 mounted in the piston, this constitution is formed so as to provide a first/second differential transformer part 5-1, 5-2 having a mutually different characteristic to be made capable of obtaining an output corresponding to a relative moving distance respectively between the cylinder 2 and the cylinder rod 4 and a cylinder displacement arithmetic part 6 calculating the cylinder displacement in the cylinder device based on the output from the first/second differential transformer part 5-1, 5-2.