A heating device for blocking flow of air if the inside of a cartridge enters a negative-pressure state is provided. The device includes a cartridge upper plate having a fluid-guide member for guiding the flow of a fluid; a cartridge lower plate having an inlet, an outlet and another fluid-guide member positioned spaced apart from the fluid-guide member of the cartridge upper plate; a heater disposed between the cartridge upper plate and the lower plate and heating the fluid being guided by the fluid-guide members; air filters positioned outside the fluid-guide members and discharging air bubbles in the heated fluid into the atmosphere; and extension units which are provided on the cartridge upper and lower plates having film members and outflow and discharge passages and that keep the flow of fluid normal in the positive-pressure state and block the fluid movement pathway if a negative pressure occurs inside the cartridge.