Time variation of the effective viscosity in the separating water-in-oil mixture was evaluated by ultrasonic spinning rheometry (USR) combining ultrasonic velocity profiler (UVP) and equation of motion. We obtained the results similar to the theoretical formula that the effective viscosity and pseudoplasticity decrease as the diameter and volume fraction of the water droplets decrease with time. On the other hand, the effective viscosity was estimated to be higher than the value estimated by theory at the initial time. One of the considerable factors is the increase in the effective volume fraction due to the accumulation of the droplets, and this requires further experiments for the evidence.