To provide an apparatus for forming a lid opening perforation in a packing container made of a synthetic resin, capable of always certainly forming two perforations and capable of also effectively preventing the damage of an intermittent blade.

A discoidal spacer plate 3 made freely rotatable and circular rotary edge tool bodies 16a, 16b, 16c, which are arranged in a freely liftable manner through an edge tool holder 13 in order to be opposed to the spacer plate, are provided and a cutting groove 4 having required width is annularly formed to the outer peripheral surface of the spacer plate 3 along its peripheral direction. Intermittent blades 17a, 17b, 17c having required thickness are formed to the outer peripheral parts of the rotary edge tool bodies 16a, 16b, 16c in order to be fitted in the cutting groove 4 in a freely slidable manner.