Abstract: Molten synthetic resin material containing foaming agent in an amount of 0.05 to 5 percent by weight is injected into a pre-pressurized mold cavity in a mold which is closed by a relatively low clamping force having a coefficient in the range of 10 to 40, said coefficient being represented as a ratio of clamping force to maximum injection shot volume. The above injection step is carried out for at least 3 seconds. After the injection step has been initiated, the pre-pressurized mold is released to atmosphere during the injection step at the soonest. At least two molds may be used to be alternately charged with said material.