A design of a spin rotator (Wien filter) is described for rotating the spin direction of positrons in RIKEN electrostatic positron system. The spin rotator will be placed between a positron gun and a parallel-plate deflector and will rotate the spin direction of positrons by +/- 180 degrees. Thus the spin direction of positrons is allowed to be polarized in longitudinal or transversal directions at the sample. However, the beam trajectories are inevitably deflected at the unmatched fringing field regions for large spin rotation angle, In this paper, we will describe the design and simulation results of three different types of spin rotators for 3 keV positrons.