Recently, various detrimental substances exhausted from a large number of cars and/or factories have been making a large influence on living environments in the large urban area. Thus, it is necessary to predict accurately the diffusion processes of such substances in the city area, which has a very complex flow field because of the existence of various buildings in size and shape. To tackle this problem, we need simplify the flow field to find the relations between mass diffusion and the flow behind a building model. In the present study, we investigate experimentally the fluid-dynamical relationship between mean velocity and concentration fields formed behind the building model by focusing attention on the behaviors of the streamwise mass flux and turbulence intensity.