To provide a hydraulic cylinder to eliminate a fear of damage occurring due to contact with an obstacle, be hardly influenced by using environment, and besides continuously detect the position of the piston.

A lift cylinder 5 comprises a cylinder tube 8; a bottom block 9; a rod cover 10; a piston rod 6; and a piston 11 formed movably integrally with the piston rod 6. An ultrasonic sensor 19 is disposed in the chamber 20 of the bottom block 9 such that the transmission side is positioned opposite to the piston 11. The ultrasonic sensor 19 is electrically connected to a transmission and receipt circuit 33 and the transmission and receipt circuit 33 to a control device 34. Based on a control signal from the control device 34, the transmission and receipt circuit 33 transmits an ultrasonic transmission signal of a given frequency to the ultrasonic sensor 19 and an analogue electric signal outputted from the ultrasonic sensor 19 is converted into a pulse signal, which is outputted to the control device 34.