PURPOSE: To obtain sufficient accuracy by obtaining a pre-load clearance from a clearance between the outer end faces of a pair of inner rings in the state of a spacer being inserted between the inner edges of the inner rings, and a clearance between the outer edges in the state of being pressed lightly in the mutually approaching direction.

CONSTITUTION: In the state of a spacer 23 not being pushed into a clearance 6, the upper face height H1 of a cover plate 34 is obtained by a measuring means 33 while pressing the outer end faces of a pair of inner rings 3a, 3b lightly in the mutually approaching direction on the basis of the weight of a hold-down block 11. The height H1 is the dimension corresponding to a clearance between the outer end faces of the inner rings 3a, 3b. With the spacer 23 pushed into the clearance 6, a clearance between the inner edges of the inner rings 3a, 3b is enlarged up to the thickness dimension T of the spacer 23. In this state, the upper face height H2 of the cover plate 34 is obtained by the measuring means 33. The height H2 is the dimension corresponding to a clearance between the outer end faces of the inner rings 3a, 3b. A pre-load clearance (h) can be obtained from h=T-(H2-H1).