Basic study is conducted on the oscillating motion of a magnetic fluid plug and a piston in a U-tube with an exciting periodic magnetic field as a simple example of the magnetic fluid plug actuator. Under the assumption that the effect of a magnetic field on pipe friction is constant, frequency response curves for the displacement of the piston are obtained by numerical calculation. Furthermore, an experimental study is carried out for comparison with the theoretical analysis. The theoretical analysis shows good agreement with the experimental results of the resonance frequency. However, prediction of the maximum amplitude requires the correction of piston seal friction.