Torque and current density on a rotating disk immersed in suspension-type electrorheological fluid (ERF) under AC electric field were measured. They have steady and varying amplitude components which depend on the angular velocity of the disk, and the frequency and intensity of the electric field. The results were demonstrated by means of a particle aggregation model. The constant torque and current density under DC electric field were compared to those under AC field. The magnitudes of the differences strongly depend on the angular velocity of the disk and the frequency of the electric field. When an AC electric field is used, a smaller frequency should be applied for the sake of greater efficiency.