A rotatable body is configured to be held to a holding surface of a holder in a non-contact state by an action of air, and a movable shaft having a same axial center as a rotation shaft. A leading end of the movable shaft directly faces an end of the rotation shaft. The movable shaft is housed in an insertion part of the holder so as to be movable in an axial direction. Centering is automatically performed by making a Bernoulli effect appear by the air flowing from a side of a first surface of the rotatable body opposite to a second surface of the rotatable body toward the second surface of the rotatable body through a space between an outer periphery of the movable shaft or an outer periphery of the rotation shaft and a peripheral edge of a mounting hole.

A rotatable body is configured to be held to a holding surface of a holder in a non-contact state by an action of air, and a movable shaft having a same axial center as a rotation shaft. A leading end of the movable shaft directly faces an end of the rotation shaft. The movable shaft is housed in an insertion part of the holder so as to be movable in an axial direction. Centering is automatically performed by making a Bernoulli effect appear by the air flowing from a side of a first surface of the rotatable body opposite to a second surface of the rotatable body toward the second surface of the rotatable body through a space between an outer periphery of the movable shaft or an outer periphery of the rotation shaft and a peripheral edge of a mounting hole.