In wet spherical agglomeration, rapid progress in the agglomeration process occurs at an almost definite elapsed time after the beginning of the agglomeration operation according to the agglomeration conditions. This rapid progress in the agglomeration process is called "phase inversion," and at this point the power number of the impeller agitating the suspension in the agglomeration tank is also known to increase rapidly. In this paper, we report our experimental investigation of the cause of the increase in the power number of the agitating impeller at phase inversion of wet spherical agglomeration of coal. The power number after phase inversion is 1.3-1.5 times that before phase inversion. At phase inversion, the agglomerate diameter increases rapidly, and the bulk volume of the agglomerates in the suspension also increases. The increase in these values greatly influences the increase in the power number at phase inversion.