A vibration mode control method for a thin plate-reinforcement structure utilizes a reinforcing effect of a weld bead, wherein the thin plate-reinforcement structure comprises two welded components at least one of which includes a thin plate portion. After a first (thin plate) component and a second (another) component are arc-welded discontinuously at several positions, a stress direction and level in one of the resulting weld areas located to be subject to stress concentration is preliminarily verified. Then, a stress-dispersing weld bead (f) is laid to extend between the weld area which is an area subject to stress concentration and the first (thin plate) component, wherein the stress-dispersing weld bead (f) is set to a direction and length capable of preventing fatigue crack from occurring in the first (thin plate) component.