An experiment on the mixing characteristics of the combustion gas in a two-dimensional test room which is a model of a marine boiler with ceiling burners has been carried out in the range of the inlet Reynolds NOs.=5x10³-3x10⁴. It was shown that the mixing characteristics in the combustion chamber were improved with an increase of flow resistance by the tube bundle and the concentration of the room, c^^-_L at the mean residence time, t_s depends on only the resistance coefficient of bundle c_f. The nondimensional concentration normalized by the inlet concentration c_o was expressed by the following empirical formula. (c^^-_L/c_o)_t=ts=0.62c_f^0.03