To prevent fatigue and deterioration of a piping welding part likely to occur in a hydraulic pipe provided with a high speed valve (causing vibration by inertia force in accordance with the motion of hydraulic fluid).

A part of pipes 4A, 4B connecting a hydraulic cylinder 1 with a hydraulic pump 2 is formed by a flexible pipe passage 5. The flexible pipe passage 5 has a double structure composed of an inner hose 51 and an outer hose 52 surrounding it. A space 53 between the hoses 51 and 52 is filled with the same hydraulic fluid as hydraulic fluid in the pipe passage. A pressure taking-out hole 67 opened in a hose space 63 is provided on a joint fitting 6 to attach a pressure detector 7 to it. The vibration of the pipes is absorbed by the flexibility of the flexible pipe 5. Even if a crack occurs in the inner hose 51 of the flexible pipe, oil leakage is prevented by the outer hose 52. When the crack occurs in the hose, a detection signal of the pressure detector 7 is outputted into an alarm machine 8 to inform a worker of the crack.