An insulator for a processing apparatus including an upper electrode, a lower electrode and a reaction chamber, the insulator being adapted to be arranged around the upper electrode and the insulator including: a bottom end adapted to face the reaction chamber; and a side wall facing a side wall of the upper electrode, wherein an edge portion of the bottom end of the insulator extends radially inwardly to form a projecting portion such that the projecting portion covers an edge of a bottom surface of the upper electrode and a clearance between the side wall of the upper electrode and the side wall of the insulator.