The present study deals with the flow field in the circular cross section of a cylindrical vessel induced by a rotationally reciprocating impeller, rotating back and forth with gradual change in rotational speed. A periodically stable velocity field was measured by PIV and also simulated by CFD. It was revealed that the velocity field near the liquid free surface is basically 2-dimensional, except for the acceleration period at Re = 43, and agrees well with 2-dimensional CFD simulation, except for the development process of 3-dimensional tip vortices at Re = 43. The separation behavior of the vortices originally generated at the impeller tip changed with increasing Reynolds number. No separation was observed at a lower Reynolds number, while the separated vortices turn into 3-dimensional potential vortices and remain as 2-dimensional vortices at the next impeller counterturn at higher Reynolds numbers. The fluctuation of each velocity component was remarkably small, suggesting that the flow field even at the highest Reynolds number was still laminar. It is found that effective transportation of energy from the impeller to 3-dimensional potential vortex is related to a large and constant power number, which is reported in our recent publication.