A reusable heating device comprises a casing to be disposed vertically, the casing including a circuit element receiving chamber and a heater receiving chamber having a front opening, a lid for at least covering the front opening of the heater receiving chamber, a heater situated in the heater receiving chamber, and electric circuit means situated in the circuit element receiving chamber. The heater includes a heater segment coextensive with and disposed in the front opening and an electric heating element attached to a backside and the cover unit of the heater segment. The heater segments have U-shaped channels on a front side thereof to receive therein a Disposable Cellular Pouch through which an instillation liquid to be heated flows. The U-shaped channels include at least left and right curves and extends from an upper portion of the heater segment to a lower portion thereof through the U-shaped channels to keep the instillation liquid in the Disposable Cellular Pouch between the left and right curves for a period sufficient to heat the liquid. The electric circuit means includes first and second temperature control thermistors, the first thermistor sensing temperature of the instillation liquid after passing through the heater segment for intermittently energizing the heater to heat the instillation liquid to a predetermined temperature. The second thermistor is provided at the electric heating element to turn off the electric circuit means when the electric heating element is heated beyond a predetermined temperature due to excessive flow of the instillation liquid to thereby override the control of the heater by the first thermistor. In this case the instillation fluid flow is not halted and will continue to flow without the heating.