PURPOSE: To roll a continuously-cast thin billet under high rolling reduction and to finish it into a hot rolled steel sheet by providing supporting rolls to the outsides of intermediate rolls in an eight-high rolling mill.

CONSTITUTION: An eight-high rolling mill is constituted of a pair of upper and lower work rolls 1a, 1b supported by each two pieces of upper and lower intermediate rolls 2a, 2b and 2c, 2d and a pair of backup rolls 3a, 3b. Supporting rolls 4a, 4b and 4c, 4d are installed at the outsides of the rolls 2a, 2b and 2c, 2d, to restrict the rolls 2a, 2b and 2c, 2d to move back and forth in the rolling direction. A continuously cast thin billet can be finished to a hot rolled steel strip by a simple rolling line because the billet can be rolled under high rolling reduction by a single stand having said construction. Further, the temperature drop of a rolling material can be controlled to a minimum, and the remarkable reduction of a consumption of heating energy can be achieved.