It is aimed to improve the heating efficiency of a poured raw material by efficiently creating a natural convection in a container. The interior of a container is constructed to have such a closed loop shape that a heating medium can be vertically circulated. A heated gas passage includes a lower heating section for heating the heating medium in a lower storage, and a lateral heating section for heating the heating medium, which was heated in the lower storage, in a first lateral section. A raw material is introduced to the heating medium which is flowing toward the lower storage in a second lateral section by being heated in the first lateral section, thereby promoting the circulation of the heating medium. In heat transferring pipes, the raw material is boiled to create drive forces for circulating the heating medium.