The present invention provides a gas generator and comprises an electrolysis device, a water supplying device, and a liquid level detector. The electrolysis device is configured for electrolyzing water to generate hydrogen. The water supplying device is coupled to the electrolysis device for supplying the supplementary water into the electrolysis device. The liquid level detector is coupled to an outer surface of the electrolysis device for detecting a liquid level of the electrolyzed water, wherein the gas generator supplies supplementary water into the electrolysis device according to the liquid level detected by the liquid level detector. The present invention is provided for measuring the liquid level by using the non-contact liquid level detector, and supplying supplementary water into the electrolysis device according to the liquid level to ensure that the electrolysis device contains sufficient water for electrolyzing, thereby improving the life and safety of the gas generator.