Steel-concrete composite decks are increasingly applied for highway bridges to reduce the initial cost of construction. In Robinson type, steel plate and concrete are integrated with studs, but that studs should be careful of fatigue damage due to repeated loading of traffic wheel loads. Because the studs subjected with rotating shear force by running wheel loads, fatigue crack occurs at welding toe of the stud. In this paper, the authors experimentally investigated the progress of fatigue cracking in composite deck using infrared thermography. From the results of infrared stress measurement, the tendency of fatigue crack progress was made clear.