A method of moulding foamed synthetic resin pellets of polyolefine in a mould, wherein the synthetic resin pellets are prepared with volumne of closed cells being more than 65 % of the total volume of the total cells and having an inorganic gas in the cells at a pressure higher than 1.18 atmospheres, then filled in a mould which is not completely gas tight, and then heated to expand and tightly adhere to each other. The pressure in the mould can be lower than ordinary pressure or at a vacuum and the pressure of the resin can be at a pressure higher than ordinary pressure. The moulded product may be removed from the mould after cooling or during cooling when the inner temperature of the product is between its melting point and a temperature 50.degree.C below the melting point. The obtained product may be stored at a temperature of 30.degree. to 70.degree.C below the melting point.