A large plate impeller was rotated back and forth in a cylindrical vessel. Simultaneous and continuous measurements of the rotational torque on the mixing shaft and impeller angular position were carried out. The power consumption was computed by integrating the product of the measured torque and impeller rotational speed. The relationship between the power number and Reynolds number revealed that the Reynolds number became small for the transition in power characteristics and that the constant power number became large for the larger Reynolds number region at a small reciprocating amplitude with a large impeller acceleration. This is because a more remarkable startup flow was frequently repeated. The larger power number suggests that the rotationally reciprocating impeller has more efficient power characteristics compared to common rotational impellers. In terms of the torque variation, the measured torque was mainly dominated by the viscous force in the smaller Reynolds number region. As the Reynolds number increased, the phase lag between the torque and impeller speed increased even in the smaller Reynolds number region, and the measured torque was observed to fluctuate in the transition region. Although the measured torque was mainly dominated by the inertia force, the phase lag and magnitude of fluctuation did not increase any further at large Reynolds numbers.