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Obtaining the reverberation time of a multipurpose building is most effective when accurate data is used to simulate the building. Therefore, this study proposes a method of measuring the sound absorption coefficient that is close to the sound absorption performance of the conditions in which building materials are actually used. In addition, a sufficient diffusivity evaluation method for sound absorption coefficient measurement in a reverberation chamber is proposed, to address the sound absorption performance difference caused by internal diffusion of the reverberation chamber. When the sound absorption performance was evaluated after installing the specimen under the condition of minimized edge effect, the result obtained should closely match the sound absorption performance of the specimen surface. The sound absorption performance of the specimen ( ${\alpha }_{\beta E\approx 0}$ ) with minimized edge effect and the sound absorption performance on the specimen surface $\left({\alpha }_{\infty }\right)$ were proposed as an evaluation indicator of agreement between the values. Experimental results show that diffusion inside the reverberation chamber is enhanced when ${\alpha }_{\infty }-{\alpha }_{\beta E\approx 0}$ < 0.02, for which sufficient diffusion can be assumed inside the reverberation chamber. In addition, to verify the validity of the proposed evaluation indicator, we investigated the relationship with the objective diffusion evaluation indicator for diffuse field configuration in the reverberation chamber, such as relative standard deviation of decay rate ( $S_{rel}$ ) and N_p values. The results of this study are expected to contribute to a more accurate estimation of the sufficient diffusion condition in the reverberation chamber, in evaluating the sound absorption performance of the material, and that inside the reverberation chamber. Full article