The oil pan structure of the present invention comprises an oil pan separator 3, disposed within an oil pan 2, for separating a main chamber provided with a suction port disposed therewithin and a sub chamber provided with no suction port from each other, the oil pan separator 3 having a recess forming the main chamber, the main chamber communicating with the inside of an engine block 1, the recess being formed with communication holes 3c, 3d for communicating the main and sub chambers to each other. The sub chamber is formed so as to surround the main chamber as a whole, whereas the outer face of the bottom part of the oil pan separator 3 and the inner face of the bottom part of the oil pan 2 are not in contact with each other. Therefore, raising the temperature of engine oil within the main chamber earlier can lubricate the engine more effectively.