To easily manufacture a form at a low cost by constructing anchor section forms different in diameter with circular arc-shaped forms constituting a synthetic resin circular form, i.e., the circular arc-shaped forms obtained by uniformly dividing the prescribed circular form into a proper number in the peripheral direction.

The resin circular arc-shaped form 1 formed for a medium- diameter anchor section is uniformly divided in the peripheral direction to form circular arc-shaped forms 2, each form is used as a unit form, and the number of the forms to be connected in the peripheral direction is increased or decreased. The unit forms 2 are circularly connected in the peripheral direction with spacer forms 3a having the proper width inserted between the unit forms 2, thereby a nearly circular form face 6 corresponding to the diameter of the anchor section to be constructed is constructed by using this synthetic resin form for constructing the anchor section.