To eliminate a hunting phenomenon when a relief flow passage is closed and eliminate the hunting phenomenon without requiring large modification to an existing unload valve.

The unload valve 10 includes: a first pilot flow passage 22a; a first block 20 formed with a first pilot flow passage 22; a second block 70 formed with a second pilot flow passage 72; a first relief flow passage 76 and a second relief flow passage 78; and a third block 100 formed with a chamber 114. A plunger 80 is provided in the first relief flow passage 76 and an orifice 132 is provided in a pilot pipe line 130. With the provision of the chamber 114 and the orifice 132, oil hammer (oil pressure variation wave) caused by close movement of the plunger 80 and propagated to a pressure oil supply pipe line 120 and the pilot pipe line 130 can be inhibited. The chamber 114 can be easily installed only by mounting the third block 100 to the existing unload valve.