To increase the amount of absorption of heat in a shell and tube once-through boiler adapted such that combustion gas is flowed vertically to a combustion gas passage provided between conduit lines of inner and outer two lines.

A shell and tube once-through boiler is adapted such that an annular upper header 1 and a lower header 2 are coupled therebetween through an inside conduit 3 and an outside conduit 4, and a combustion gas passage 7 is provided between the inside conduit and the outside conduit, through which passage combustion gas is flowed vertically. In the boiler, heat absorption fins 11 of multiple stages are provided at a portion facing the combustion gas passage 7 between the inside conduit 3 and the outside conduit 4 so as to intersect a combustion gas flow. The heat absorption fins 11 are provided at the same heights for the inside conduit 3 and the outside conduit 4, so that there are provided many stages of contraction sections having a cross sectional area of the combustion gas passage with the heat absorption fins 11.