Abstract: An oil tank including an oil tank body for storing oil and an oil filter disposed between an oil inlet for allowing oil into the oil tank body and an oil outlet for sending oil out. An oil inlet chamber is formed, inside the oil tank body, from a portion of the side wall, a portion of the bottom wall and the inner wall of the tank body, and communicates with the oil inlet. The oil filter has a substantially flat shape and forms a portion of the inner wall. By the use of the oil filter having a reasonably large mesh area, independently of the diameter of the oil inlet, the oil filter is protected against a mesh damage and is prevented from being detached. Since no excessively large area mesh is required, no particular rise in material costs is expected. A substantially flat surface structured oil filter is easy to fabricate and less costly to fabricate as well.