In the washing process of the invention, the solid particles in a high-concentration zone, which is formed in a washing tank by a gravitational sedimentation of solid particles, are continuously washed by a counter-current contact with upward flow of a washing liquid which is fed from the bottom portion of the washing tank. With this process, the impurities in the solid particles are sufficiently removed by a simple apparatus. Since the used washing liquid can be recycled as the disperse medium for feeding the solid particles and as the washing liquid, the amount of used washing liquid to be discharged as the waste from the system is reduced.