Abstract: An amount of scale adhering onto inner surfaces of water tubes in any one of various boilers including a steam boiler, a hot water boiler, a waste heat boiler, and an exhaust gas boiler is accurately known by providing a temperature sensor at an appropriate position of the boiler. A plurality of water tubes (5, 6) connecting between an upper header (3) and a lower header (4) constitute an inner water tube row (7) and an outer water tube row (8). Inner longitudinal fins (9) close gaps between the adjacent inner water tubes (5, 5) except at a lower end of the inner water tube row (7). Outer longitudinal fins (11) close gaps between the adjacent outer water tubes (6, 6) except at an upper end of the outer water tube row (8).