To provide the head of an accumulator type blow molding machine which can form a good annular parison by efficiently preventing a failure caused by the burning of a molten resin put into the clearance between the inside surface of a cylinder and the outside surface of a piston.

On the outside surface 26 of the annular piston 22 facing the inside surface of a housing, an annular projected part 27 for seal surrounding the periphery of the piston 22 is formed in the lower part of the piston 22. Flow hindering projected parts 28 are formed to be separated from each other at positions above the annular projected part 27 so that the lower surfaces 29 of the projected parts 28 are formed in the accurate shape protruding to the lower side of the piston 22 or approximately in a V-shape.