The launch vehicle with a liquid propulsion system requires pressurization during the propellant discharge. While the launch vehicle is flying through the air layer, a process in which frictional heat is generated between the liquid propellant tank and the air layer can be assumed. External heat inflow occurs during the liquid discharging process, and it is necessary to investigate how the external heat influx affects the liquid pressurized discharge process. A test device was constructed to determine the effect of pressurization of external heat inflow. The test device consisted of a liquid tank located in a vacuum chamber and a heating device to simulate external heat inflow. The heating device is a halogen lamp. The cryogenic liquid discharge with and without external heat inflow was compared. It was confirmed that the amount of pressurant gas used was decreased when there was external heat inflow. External heat inflow contributes to the evaporation of the liquid, and the evaporated gas increases the pressure of the ullage.