A magnetic field shifter for the evaluation of garnet film which is used as a readout head for magnetic media has been developed. The shifter enables to generate a sinusoidal magnetic field along one direction with a magnitude of up to 60 Oe and to move the magnetic field at any speed from few cm/s to 80 m/s. The garnet film to be evaluated was placed in close contact to the field shifter and output signals from magneto-optic detection system were measured. An obtainable carrier-to-noise ratio was dependent upon transfer modes which took place in a garnet film. The speed response in transfer readout could not be revealed. This may be ascribed to the fact that the effective wavelength of shifting magnetic field is too long (140 μm).