Single crystals of Rb2ZnCl4 were grown by slow evaporation of an aqueous solution which had been purified by repeated recrystallization. Special care was taken to remove impurities having a tendency to enter into crystal in crystallization. The dielectric constant of the purified crystal showed thermal hysteresis of only 0.2 K in the incommensurate phase. The dielectric dispersion frequency of domain wall motion in the commensurate phase was lowered to 200 kHz from 1 MHz in the unpurified crystal. The switching time of the field-induced commensurate-to-incommensurate transition was shortened extremely in the vicinity of Tc. These indicate the weakening of the pinning effect in the purified crystal.