A heat sensor includes a base and a heat sensor body. The base is to be mounted onto a mounting surface of a building. The heat sensor body has a bottomed cylindrical shape and is to be attached to the base. The heat sensor body includes an opening, a board, a heat detection unit, and at least one wall member. The at least one wall member controls flow of a gas to cause the gas that passed through the opening to flow toward the heat detection unit. The at least one wall member separates the flow of the gas that has entered the heat sensor body from an external space through the opening into a plurality of gas flows and directs one of the plurality of gas flows, which has been separated to flow beside an inner surface of the heat sensor body, toward the heat detection unit.