To cool hydraulic oil in a heated hydraulic cylinder.

This hydraulic cylinder of small capacity type, having a piston rod 5 sliding in the axial direction, piston 10 connected to the piston rod 5 to slide in a cylinder tube 13, two oil chambers 7, 8 formed to be partitioned by the piston 10 to supply hydraulic oil to and discharge it from a connection pipe, and a seal member 6 provided in the cylinder tube 13 to prevent a leak of oil, is used under a high temperature environment, and a leak path 9 connected so as to generate a small amount of leak oil is provided between the oil chambers 7, 8.