Live and dead loads, effects of drying shrinkage and creep of concrete and relative difference of temperatures of steel and concrete are considered in design of steel-concrete composite deck bridges. At Narita international airport, firm measurement of strains and temperature is ongoing by gauges embedded in the thick concrete slab This paper describes the considerations about the generation status and generating factors of thermal stresses based on calculation from measured data of the actual bridge, and makes certain that the distributions of thermal stresses are affects to be taken into account in design of steel- concrete composite bridges with thick slabs.