PURPOSE: To prevent an occurrence of a craze or a crack in the molding of small openings by a method wherein a piercing member having hole-forming protrusions of the required number is provided in a lower mold, the protrusions are pushed up to be projected from the lower mold, a molded piece with intermittent small openings is obtained by clamping the molds after the supply of a melt resin, and the piercing member is lowered before the molded piece is cured.

CONSTITUTION: A lower mold 12 is provided with a piercing member 4 for forming small openings 3 in a molded piece 2; a lifting device 5 for lifting the piercing member 4; and an extracting member 6 for lowering the piercing member 4. On the top surface of the piecing member 4, hole-forming protrusions 8 of the required number and a required shape are provided for forming the intermittent small openings 3 of the molded piece 2. The hole-forming protrusions 8 of the piercing member 4 are projected from the lower mold 12 by the lifting device 5. After a melt resin is supplied between an upper mold 11 and the lower mold 12, the molds are clamped for molding the resin into the piece 12 with the intermittent small openings 3. Before the molded piece 2 is cured, the hole-forming protrusions 8 are lowered by engaging the extracting member 6 to the piercing member 4.