The basic fatigue strength of welded joints, which corresponds to that of the real large scale welded structures containing high tensile residual stresses of yield strength magnitude, was investigated by the maximum stress constant test. The plate thickness effect was revealed to be an imaginary phenomenon occuring from the small residual stresses induced by a variation in maximum stress with the applied stress. The Miner's law was applicable for the basic fatigue strength. The basic fatigue strength increased with a decrease in yield strength.