This is a micro-ethnographic study focusing on a traditional custom at Tōshi Island in Japan. When first-born sons in the island graduate from junior high school, they form a small group of neya-ko (quasi-brothers) and sleep over at neya-oyas’ (quasi-parents) house every night until they become 26 years old. They keep the quasi-family relationship alive and help each other all through life. We investigated the maintenance process of this cultural custom by participant observations and unstructured interviews. Because of the recent drastic environmental changes around the island, people face difficulties maintaining the custom. In spite of these circumstances, they have been able to maintain the custom not by challenging the environmental changes, but by constantly changing the custom itself to fit the environment. Their flexible decision making may derive from their ecological basis as fishermen. Based on the findings, we discuss the sustainability of cultural customs facing environmental changes.