The investigation focused on the belief in the monitor lizard spirits of the Kui ethnic group in the study area. The ethnic residents have been involved in rice cultivation, so they are not widely known as those tending and training elephants. It has also been revealed that the ethnic group believed that monitor lizards were their ancestral spirits. When they settled in the village, monitor lizards were abundant. Originally, they believed that wild boars were their ancestral spirits. When wild boars became scarce, they turned to monitor lizards as their ancestral spirits. The investigation revealed that the belief in monitor lizards was only a part of their lifestyle, for instance, designs of the ethnic textile weaving or community forest conservation. The belief is beyond other beliefs of the ethnic group and it is a strong collective belief in conjunction with the threat of supernatural power. For instance, if the villagers were not united, the spirits might be displeased and cast a spell of drought on the village. Consequently, the belief could facilitate peaceful and orderly co-habitation of the villagers, e.g., agreements on the exploitation of community forest or natural resource conservation. The belief played a part in the language and cultural conservation of the ethnic group amidst modern trends of the present society.