To provide an apparatus capable of expanding the diameter of an inner cylinder in which the inner cylinder of an isolator is stably supported and the inner cylinder is accurately expanded into a desired size.

This apparatus is constituted so that a first supporting body 10, an annular die 20 and the second supporting body are provided, the die 20 is divided into a plurality of divided dies 21 and the plurality of the divided dies 2 are supported with the second supporting body so that their positions are freely changeable. The inner peripheral parts 21N of the plurality of the divided dies 21 are pressed with a diameter expanding punch P with the diameter expanding punch P which is inserted into the inner cylinder 1 placed on the inner cylinder placing part from the side of the upper part Z2 is projected with the punch projected from the inner cylinder 1 while expanding the diameter of the inner cylinder 1 and the plurality of the divided dies 21 are constituted so as to change the positions to the side of the second stroke end with the inner cylinder 1 placed on the inner cylinder placing part.