PROBLEM TO BE SOLVED: To prevent any further hydraulic pressure rise of a tank circuit and a drain circuit, and to protect the drain circuit of a hydraulic motor by automatically closing (shutting off) a hydraulic pressure feed circuit. SOLUTION: A shutoff valve 31 is disposed in a hydraulic pressure supply circuit 22 from a tractor to hydraulic motors 16a, 16b via a coupler P. A hydraulic pressure sensitive shutoff spool 31a to be operated by the hydraulic pressure when the hydraulic pressure of the tank circuit 24b exceeds the predetermined value is disposed in that tank circuit 24b returning from the hydraulic motor to the tractor via a coupler T. The shutoff valve 31 is closed by the operation of the shutoff spool 31a having a hydraulic pressure sensitive member. COPYRIGHT: (C)2012,JPO&INPIT
