PURPOSE: To completely separate muds interposed between projecting parts and taken up by forming films between adjacent fin-shaped projecting parts.

CONSTITUTION: At a grounding position W1, muds S make ingress between lugs 4 and 4 and fin-shaped projections 5 and 5. In a band W2 where the muds are wound around a rolling wheel 9 and convert their direction, the space between the lugs 4 and 4 is enlarged so that the muds held therebetween are exfoliated from the side walls of the lugs and fall by losing their supports. While rubber films 6 extending between the fin-shaped projections 5 and 5 are in a curved state by the infiltration pressure of the muds S in the band of the grounding position W1, the rubber films elongate rectilinearly in the nongrounding position W2. As a result, a force for extruding from the backward is applied to the muds S, and the muds S are positively purgeed from the space between the lugs towards the direction shown by arrow P.