A bell mouth includes an upstream body including an upstream-side opening formed to allow air introduced from an air supply duct to pass through and an upstream measurement portion connected with a differential-pressure acquisition device that uses pressure of the air in the upstream-side opening, in which the air supply duct guides the air into a water-heating device from the outside, a differential-pressure generation part that is located downstream of the upstream body with respect to a flow direction of the air and that has a structure that reduces the pressure of the air passing through the differential-pressure generation part, and a downstream body including a downstream-side opening formed to allow the air passing through the differential-pressure generation part to pass through and a downstream measurement portion connected with the differential-pressure acquisition device that obtains a difference between the pressure of the air passing through the downstream-side opening and facing toward a blower of the water-heating device and the pressure of the air in the upstream-side opening.