To easily and quickly remove self-sealed foam scum, easily vent, to prevent action from becoming bad, to decrease amt. of use of raw materials for a foam, to improve yield and to decrease the number of vent holes.

This structure provides a cylinder structure for a vent hole 5. The cylinder structure is provided with a cylinder main body 13 and a movable rod 15. A valve element 21 with a vent channel 22 is provided on the bottom part of the movable rod 15. A scum scraping part 25 for the vent hole 5 is provided on the top part of the movable rod 15. The vent channel 22 of the valve body 21 is formed by processing the face 22a. When a foamed body is removed from a mold after it is molded, the cylinder mechanism 11 is actuated and the scum scraping part 25 in the vent hole 5 is pushed in the top mold 3 to remove scum 7c of the foamed body. The scum 7c of the foamed body in the vent channel 22 of the valve body 21 is removed by utilizing the face 22a.