In oxygen therapy, high-flow oxygenation devices are employed for patients with respiratory failure. Heating and humidification are essential for high-flow oxygenation. In this study, we develop a system that can quantitatively and objectively monitor heating and humidification status in high-flow oxygenation devices. The system consists of a temperature and humidity sensor, a microcontroller, a piezoelectric speaker, and LEDs. It is independent of the high-flow oxygen apparatus and is powered by a battery to monitor the status without being affected by other equipment. We measured the temperature and humidity values of the gas supplied from the high-flow oxygenation device to the oxygen mask over time. The temperature and humidity values confirmed the presence of sterile water and the operation of a heater. The humidity and temperature values decreased in the absence of sterile water and heater, respectively. In addition, by setting a threshold for the temperature and humidity values, we could quantitatively and objectively grasp abnormal conditions. Under an abnormal condition, the LEDs emit light and an alarm is issued; this enables visual and auditory confirmation of the situation and prompts response to the abnormal condition.