Axially extending liquid-tight tapered spaces are defined in the barrel of a roll body. Tapered pistons are fitted in the spaces for engagement and disengagement with a bore inner surface defining the tapered spaces. A liquid under pressure is supplied through liquid passages to some of liquid pressure chambers at opposite ends of the tapered pistons so that the barrel of the roll body is increased or decreased in diameter. When the rolling forces are removed from the roll body, the roll body returns to its initial straight state.
