A method of producing a thin plate-reinforcement structure utilizing a reinforcing effect of a weld bead, comprising: - a first step of preparing a first component including a thin plate portion, and a second component having a thickness greater than that of the thin plate portion, the first component being right and left side frames of a back frame to be installed inside a seat back of an automobile seat, each of the right and left side frames being composed of two superimposed plates, the second component being a reinforcing member to be bridgingly arc-welded to the right and left side frames; and - a second step of welding the first component and the second component together, characterized in that the second step includes the sub-steps of: ∘ bringing the reinforcing member into contact with an obverse surface of a laterally protruding portion of a respective one of the right and left side frames, ∘ welding the first component and the second component together through at least two weld areas by arc-welding the reinforcing member to the side frame from a side of a reverse surface of the laterally protruding portion of the side frame; and ∘ providing a stress-dispersing weld bead (t, u) to extend from an overlapping region between the weld area which is an area subject to a stress concentration and the reverse surface of the side frame, onto the first component wherein the stress-dispersing weld bead (t, u) is set to a direction and length capable of preventing fatigue crack from occurring in the first component. - a first step of preparing a first component including a thin plate portion, and a second component having a thickness greater than that of the thin plate portion, the first component being right and left side frames of a back frame to be installed inside a seat back of an automobile seat, each of the right and left side frames being composed of two superimposed plates, the second component being a reinforcing member to be bridgingly arc-welded to the right and left side frames; and - a second step of welding the first component and the second component together, characterized in that the second step includes the sub-steps of: ∘ bringing the reinforcing member into contact with an obverse surface of a laterally protruding portion of a respective one of the right and left side frames, ∘ welding the first component and the second component together through at least two weld areas by arc-welding the reinforcing member to the side frame from a side of a reverse surface of the laterally protruding portion of the side frame; and ∘ providing a stress-dispersing weld bead (t, u) to extend from an overlapping region between the weld area which is an area subject to a stress concentration and the reverse surface of the side frame, onto the first component wherein the stress-dispersing weld bead (t, u) is set to a direction and length capable of preventing fatigue crack from occurring in the first component. ∘ bringing the reinforcing member into contact with an obverse surface of a laterally protruding portion of a respective one of the right and left side frames, ∘ welding the first component and the second component together through at least two weld areas by arc-welding the reinforcing member to the side frame from a side of a reverse surface of the laterally protruding portion of the side frame; and ∘ providing a stress-dispersing weld bead (t, u) to extend from an overlapping region between the weld area which is an area subject to a stress concentration and the reverse surface of the side frame, onto the first component wherein the stress-dispersing weld bead (t, u) is set to a direction and length capable of preventing fatigue crack from occurring in the first component.