The present invention provides a thermal liquefying apparatus as well as a thermal liquefying method, having an improved efficiency of heating the inputted material by efficiently causing natural convection in the vessel. The inside of the vessel (2) is so constituted in a closed loop that a heating medium (7) vertically circulates. A heating gas passage (5) comprises a lower heating section (21)for heating the heating medium (7) in a lower reservoir section (11), and a side heating section (22) for heating the heating medium (7) in a first side section (12) heated in the lower reservoir section (11). The material (1) is heated in the first side section (12) and introduced into the heating medium (7) flowing through a second side section (13) toward the lower reservoir section (11) and accelerates circulation of the heating medium (7). In the heat exchanger tube (14a), the material (1) boils to generate a circulation driving force of the heating medium (7).