PURPOSE: To obtain the thermostatic apparatus having high accuracy comprising an inverted bell-jar shaped container capable of flowing a fluid through the space of the double wall thereof, capable of using a gas as a direct heating medium and capable of disharging and introducing a matter to be treated without disturbing the temp. equilibrium condition of a system.

CONSTITUTION: An inverted bell-jar shaped container as the aforesaid thermostatic apparatus is constituted form double walls 1, 2 and a space 3 for passing a heating fluid medium comprising a liquid or a gas is provided between said walls 1, 2. In addittion, a piston type pump 8 is provided in order to sent and supply the fluid and a heater 9 consisting of a heat exchange part and a heat generating part is arranged as well as a sensor 10 for measuring the temp. of the fluid is further provided. In addition, a controller 11 for regulating the temp. of the fluid to a predetermined correct temp. corresponding to the sensor 10 is provided. This controller 11 constituted frm a part setting a temp. to a determined valve, a part converting output of the sensor 10 to an ON-OFF signal and a part operating and cutting off a heat source. A figure 4 represents thermostatic space part.