To make a burr mark inconspicuous while facilitating the treatment processing thereof.

In a pair of blow molds having cut-off blades 21 provided to the peripheral part of a cavity C, edge parts 22 coming into contact with each other by mold clamping are formed to both cut-off blades 21, 21 and, further, the expansion parts 23 protruding only the edge parts 22 of one cut-off blades 21 toward the cavity are formed so as to provide a stepped part 8 to the parting line part 24 on the cavity side.