To protect a coupler of a working roll of a rolling mill from water droplets and dust floating around the rolling mill by arranging a dust seal between a drive shaft of a driving device to drive the working roll and the working roll.

A lip part make contact with end surfaces of couplers 50a, 50b while it further opens and a space between drive shafts 52a, 52b to rotationally drive a working roll, more in detail, a space between roll shafts 28a, 32a and the couplers 50a, 50b is blocked from outside when fitting projections 28b, 32b are fitted to fitting holes 56a, 56b of the couplers 50a, 50b after a dust seal 10 is fixed on head end parts of the roll shafts 28a, 32a on the driving side of the working roll. Consequently, it is possible to prevent intrusion of water droplets and dust floating in the circumference of a rolling mill in the couplers 50a, 50b, more in detail, between the fitting projections 28b, 32b and the fitting holes 56a, 56b and to prevent deterioration and outflow of grease between the fitting projections 28b, 32b and the fitting holes 56a, 56b.