To provide a center joint which has a structure eliminating labor for connection and separation of lower piping and upper piping accompanied by fuel supply and damage to piping and the like caused by forgetting of the separation while eliminating a risk of mixing of a hydraulic oil into the fuel, and to provide a working machine using the center joint.

A rotary joint for the hydraulic oil comprises an external cylindrical body 42 and an internal rotating body 43. A fuel passage 53 is provided in a shaft center of the internal rotating body 43. A fitting portion 43a of the lower end of the internal rotating body 43 and a fitting portion 59 provided in a lid body 45 are rotatably fitted. A ring-shaped seal member 60 is provided between the fitting portions 43a, 59. The lid body 45 fixed to the undersurface of the external cylindrical body is provided with a ring-shaped drain groove 56. A drain discharge port 62 is provided for communicating with the drain groove 56. Even when the seal member 60 is damaged to cause leakage of the hydraulic oil, the hydraulic oil is discharged from the drain groove 56 through the drain discharge port 62, so as to prevent mixing of the hydraulic oil into the fuel.