To provide a furnace wall structure for boiler which is improved in reliability or from which the need of maintenance is eliminated by preventing the occurrence of cracks in the toe section, particularly, the heat transfer tube-side toe section of a membrane bar existing in the coupling section, etc., between the heat transfer tube of a furnace wall and the membrane bar.

When the inside diameter t of the heat transfer tube 1a connected to the thicker part side of the membrane bar 3 and the inside diameter t0 of another heat transfer tube 1b connected to the thinner part side of the bar 3 are adjusted to meet the relation of t>t0, the stress which occurs in the junction of the tube 1a with the bar 3 becomes lower as compared with another membrane bar 2 having the same thickness and the stress distribution in the bar 3 becomes relatively uniform throughout the bar 3. Accordingly, the reliability of the portion is improved. In addition, when the thickness of the end face of the bar 3 is set to an appropriate value, the portion where the maximum stress occurs in the bar 3 moves from the end face side to the central part. Consequently, the occurrence of cracks in the boundary section 16 between the heat transfer tube 1 and membrane bar 2 is eliminated and, the damages to members can be evaluated extremely easily, because the inspections of the members can be finished by only inspecting the central part of the membrane bar 3.