PURPOSE: An airtight structure of a reducer is provided to secure safety of a reducer by comprising an airtight means which blocks outside water from flowing into the inside of a housing. CONSTITUTION: A case accommodates gears. An input shaft(110) and an output shaft(120) penetrate the case for installation. Multiple bearings(300) movably support the input shaft and the output shaft. The multiple bearings are installed at the case. A ring member(500) is combined at the outer circumference of the input and output shafts. The ring member rotates with the input and output shafts. A bearing housing(400) is separated from the ring member at a microgap. The bearing housing is fixed to the case. An airtight means(600) is formed between the bearing housing and the ring member. The airtight means prevents lubricating oil from leaking to outside or water of outside from flowing into the case. The airtight means comprises a first sealing member(620).
