To provide a pressure reducing valve that malfunction resulting from complicated pressure control is evaded and the extinguishant in an extinguishant tank is used up.

This pressure reducing valve has a high-pressure flow passage 10 applied with supply gas pressure P1, a low-pressure flow passage which connects with the high-pressure flow passage 10 at a flow-passage mouth 20 and is applied with sent-out gas pressure P21, a valve body which is arranged in a flow plate 43 fixed in the high-pressure flow passage 10 so that it freely moves forward and backward, and a disk spring 50 which is arranged on the side of the low-pressure flow passage 30 and abuts no the valve body 40 and energizes it to leave the flow-passage mouth 20.