To provide a float valve capable of opening a valve port with a large opening area when differential pressure between an inlet and an outlet is small, and capable of opening a valve port with a small opening area when the differential pressure between the inlet and the outlet is large.

The exhaust valve is provided with a valve casing 1 having first and second openings 2 and 3 used as the inlet or the outlet, and having a valve chamber 4 formed between both the openings, a first bulkhead member 5 separating the first opening and the valve chamber, a second bulkhead member 7 separating the second opening and the valve chamber, a first inflow port 10 and a first valve port 11 formed on the first bulkhead member, a second inflow port 14 and a second valve port 15 with as opening area smaller than the first valve port 11 formed on the second bulkhead member, a first check valve element 12 allowing the inflow of a fluid into the valve chamber from the first opening through the first inflow port, and prohibiting the outflow to the first opening from the valve chamber, a second check valve element 16 allowing the inflow of the fluid to the valve chamber from the second opening through the second inflow port, and prohibiting outflow to the second opening from the valve chamber, and a float 17 housed in the valve chamber and opening and closing the first or second valve port.