Abstract: The drain valve apparatus and an air separator are provided. The air separator includes a main body having an upper cover and a cooling shaft extended the upper cover, an oil filter coupled to the cooling shaft, an outer body coupled to the upper cover and surrounding the main body and the oil filter, a drain body disposed at the outer body and having an interior passage, a drain valve passage, an air inflow passage and an exterior passage, and a drain valve. The drain valve apparatus includes a piston that has a first body, a second body, a connecting member, and a blocking body, and an elastic member that is connected to the first body and moves along a length direction of the drain valve passage.