PURPOSE: To enable the measurement of a roll clearance without stopping rolls, in two rolls having parallel axial lines, by making one of two rollers freely approachable and separable with respect to the opposed one and measuring the position of the outer peripheral surface of one roll in the side opposite to a material passing side by a non-contact type position measuring instrument.

CONSTITUTION: When a zero point got out of order or the position of a roll 3 was altered, the position of the outer peripheral surface of roll 2 corresponding to the non-abrasion part 2a thereof is measured, in such a state that the roll 2 is contacted with the roll 3, by a non-contact type position measuring instrument 15 and, next, the positions of outer peripheral surfaces of the roll 2 corresponding to the non-abrasion part 2a and the abrasion part 2b thereof are measured, in such a state that the roll 2 is spaced apart from the roll 3, by the position measuring instrument 15. Subsequently, a roll clearance is operated from the difference between the positions of outer peripheral surfaces of the non-abrasion part 2a at the time of the contacted state and at the time of the spaced apart state and a roll abrasion amount is calculated from the difference between the positions of the outer peripheral surfaces of the abrasion part 2b and the non-abrasion part 2a at the time of the spaced apart state by operation.