A hollow injected shaped body of synthetic plastic is produced by injecting a predetermined quantity of a synthetic plastic melt into a multi-part mold, and forming the melt to the shaped body by a gas under pressure. After bringing the synthetic plastic melt into the mold cavity, the gas is injected into the synthetic plastic melt, then after overcoming the shaped body wall it flows into an overflow chamber from it into a gas circulation system so that during the forming of the melt a hollow formed in the shaped body is continuously supplied with the gas until rigidification of the shaped body.