In order to carry out accurate measurements of absorption coefficient by Sabine formula, the assumption of diffuse distribution of sound energy in the reverberation chamber must be fulfilled. Thus, this work was started from the basic researches of the sound field in the two and three dimensional model rooms. The standing wave pattern of each normal mode was investigated using the dust figures and other methods. At low frequencies, normal mode of vibration existed even when the room shape was made adequately irregular, and for each normal mode, spatial variation of sound pressure did not depend on room shape. However, as the room shape becomes irregular, the nodal lines of successive normal modes distribute at random in space. Also, in the properly irregular room, the variation of decay rate for each normal mode was much smaller than that of a rectangular room.