A crack detection (CD) paint was applied to weld lines of a transverse rib welded joint specimen of a rolled steel for welded structures, and the effects of the paint on visual detection of fatigue cracks were evaluated by performing a fatigue test. Remarkable color development was observed in the CD paint when fatigue cracks propagated along the weld toe lines. At intervals in the fatigue test, surface SH (Secondary Horizontal) wave tests were also carried out in order to confirm the existence of fatigue cracks, and the test results were compared with the color development in the CD paint.