PURPOSE: To reduce the generation ratio of a bad product, by a method wherein an auxiliary cavity is provided on the way of an air vent and the cross area and the length of a second air vent comminucated with the outer surface side of a metal mold made smaller and longer than that of a first vent communicated with the cavity side of a product.

CONSTITUTION: For example, in a metal mold for the RIM molding of a steering wheel S, an auxiliary cavity 11 is provided on the way of air vent and the cross area and the length of a second air vent 15 communicated with the side of a metal mold outer surface 14 are made smaller and longer than that of a first air vent 13 communicated with the side of a product cavity 1. An air entrapped defect part is certainly blown out into the auxiliary cavity 11 because the flow of a material in the first vent 13 is well as well as the foaming ratio in the product cavity is made high. In addition, even if the foaming ratio of a material is high, the material of a leading end part containing the defect part is limited in the blow off because the second air vent 15 restrains the material flow to be accumulated in the auxiliary cavity 11 and the material foaming ratio is suppressed and the weight of a product is not made too light.