PURPOSE: To separate and remove mud effectively from charged material by providing a washing section to a part of a vibrating screen and removing the dirt sticking to the surface of the screen utilizing friction of charged material with each other generated by vibration of the screen.

CONSTITUTION: Mixture of muddy water of high concn. transported from the site of excavation and earth and sand of rock mass is charged to a pool 3 provided to the charging section of the mixture on a screen 6 for coarse gravel from arrow mark direction A. By the vibration of a screen main body 1 to the arrow mark direction F by the effect of a low bed type vibrator 2, the mixture is diluted with washing water in the pool 3 at the charging section, affected by frictional effect between particles with each other contained in the muddy water by the vibration of the screen, and mud component sticking to the surface of inside surface of sand and gravel is removed. In this way, the sand, gravel and mud water separated from strongly adhered mud are transferred over a weir 4 to the screen for coarse gravel 6, and the coarse gravel is classified. Fine gravels are further classified by a screen for fine gravel 7, and discharged thereafter.