PURPOSE: To improve collecting efficiency of removed shell by forming a skirt bulge portion on a skirt portion in the orthogonal direction, at brush interfering portion, to a center line connecting centers of two brushes which right and left are in contrast with each other and make reverse rotation as interfering with each other at their brush ends.

CONSTITUTION: A skirt portion 11 on the center line of two rotary brushes 10, Which right and left brushes are in contrast with each other and make a mutual interference and reverse rotation, are formed with a skirt bulge portion 11a on both skirts. The size of a bulge B on the skirt bulge portion 11a is set to S≤B≤3S where S is the distance between the brush end and the skirt portion. Rising of water pressure at the central portion where sea water mixed with removed shell dashes against each other moderates and becomes equal to the water pressure on the outer side of the skirt. Thus movement of sea water mixed with removed shell from the inner side to the outer side of the skirt is eliminated, scattering of removed shell is prevented and removed shell collecting efficiency can be improved.