The invention provides a crankcase structure, which inhibits the change and descending of oil level in the bottom of the casing due to the inclination of an internal combustion engine and enables the oil to rapidly return for preventing the exposure of an oil-sucking opening and preventing an oil pump from generating cavitation. In a supporting crankshaft (3) and a speed changer (4) and a crankcase (2) of an internal combustion engine (1) having an oil sump (61) at the bottom, an oil-sucking opening (66) which sucks the oil in the oil sump (61) through an oil pump (62) is disposed at the bottom of the oil sump (61). The crankcase (2) comprises an oil return port (74) which enables the oil of a lubricating crankshaft (3) or the speed changer (4) to return to the oil sump (61), separating walls (71L, 71R) for separating the crankshaft (3) and the oil sump (61) or separating walls (72L, 72R) for separating the speed changer (4) and the oil sump (61), and oil discharging parts (78, 78') which are connected to the separating walls and are at least partially disposed at the upper part of the oil surface when the internal combustion engine storing proper amount of oil stops.