This paper describes the numerical simulation results of radiative heat transfer between the human body and an urban street canyon (building walls, pavement, and the sky) in the presence of participating non-gray gas mixtures consist of H_2O and CO_2. The ambient temperature of typical summer condition and the concentration of gas mixtures in summer Tokyo were assumed, and parallel infinite plane model and simple urban street canyon model were used. The results show that the participating gas significantly affects the infrared radiation field in urban street canyon. The radiation flux emitted by the participating gas is approximately 35% of the total incident radiation flux to the human body surface. This causes a homogenization of the infrared radiation field. Gas radiation plays an important role in the heat transfer between human and environment under hot and humid summer condition.