A manufacturing method of a synthetic resin hollow body in which a molten resin is injected for an over-molding outside a hollow molding body made of a resin provided with a hollow body and at least an opening portion to form a resin sheathing body in an integrating manner with the hollow molding body. The method includes the step of injecting the molten resin for an over-molding under the state in which the hollow molding body is partially or wholly filled with a fluid substance. The manufacturing method of a synthetic resin hollow body is capable of suppressing resin cost and operation cost and can improve decorating property.