An oil seal constructed by a combination of a lip seal member which is adapted to be fixed to an inner periphery of a shaft hole of a housing and a slinger which is adapted to be fixed to a rotary shaft inserted to said shaft hole, and inhibiting a dust in an external side from entering into a machine inner side as well as inhibiting a sealed fluid in the machine inner side from leaking to the external side, the oil seal comprising: a dust cover together with said lip seal member and said slinger, wherein said slinger is integrally provided with a tubular portion which is adapted to be fitted to said rotary shaft and a flange portion which is formed so as to rise up toward an outer side in a diametrical direction from said tubular portion, the oil seal being characterised in that said flange portion of said slinger is provided with a thread groove which carries out a pumping action toward an outer side in a diametrical direction in an end surface in an external side of said flange portion, wherein said dust cover is integrally provided with a cylinder portion which is fitted to the tubular portion of said slinger and a diametrical portion which is formed so as to rise up toward an outer side in a diametrical direction from said cylinder portion, said diametrical portion is provided with a structure which achieves a dust shake-off action on the basis of a centrifugal force at the shaft rotating time, and a labyrinth seal action by forming a micro gap in relation to said lip seal member, wherein said lip seal member is provided with a main lip which slidably comes into close contact with the end surface in the external side of the flange portion in said slinger so as to seal said sealed fluid, and a dust lip which slidably comes into close contact with an outer peripheral surface of the tubular portion in said slinger so as to suppress intrusion of said dust as well as being arranged between said main lip and said dust cover, and wherein said dust lip is constructed by a rubber-like elastic body and is integrally provided with a first bellows portion which receives an internal pressure between said dust lip and said main lip by an outer peripheral surface as well as receiving an atmospheric air pressure in the external side by an inner peripheral surface, a second bellows portion which receives the internal pressure between said dust lip and said main lip by an inner peripheral surface as well as receiving the atmospheric air pressure in the external side by an outer peripheral surface, and a lip end which slidably comes into close contact with an outer peripheral surface of the tubular portion in said slinger, and a pressure receiving area is set to be greater in said first bellows portion than in said second bellows portion. a dust cover together with said lip seal member and said slinger, wherein said slinger is integrally provided with a tubular portion which is adapted to be fitted to said rotary shaft and a flange portion which is formed so as to rise up toward an outer side in a diametrical direction from said tubular portion, the oil seal being characterised in that said flange portion of said slinger is provided with a thread groove which carries out a pumping action toward an outer side in a diametrical direction in an end surface in an external side of said flange portion, wherein said dust cover is integrally provided with a cylinder portion which is fitted to the tubular portion of said slinger and a diametrical portion which is formed so as to rise up toward an outer side in a diametrical direction from said cylinder portion, said diametrical portion is provided with a structure which achieves a dust shake-off action on the basis of a centrifugal force at the shaft rotating time, and a labyrinth seal action by forming a micro gap in relation to said lip seal member, wherein said lip seal member is provided with a main lip which slidably comes into close contact with the end surface in the external side of the flange portion in said slinger so as to seal said sealed fluid, and a dust lip which slidably comes into close contact with an outer peripheral surface of the tubular portion in said slinger so as to suppress intrusion of said dust as well as being arranged between said main lip and said dust cover, and wherein said dust lip is constructed by a rubber-like elastic body and is integrally provided with a first bellows portion which receives an internal pressure between said dust lip and said main lip by an outer peripheral surface as well as receiving an atmospheric air pressure in the external side by an inner peripheral surface, a second bellows portion which receives the internal pressure between said dust lip and said main lip by an inner peripheral surface as well as receiving the atmospheric air pressure in the external side by an outer peripheral surface, and a lip end which slidably comes into close contact with an outer peripheral surface of the tubular portion in said slinger, and a pressure receiving area is set to be greater in said first bellows portion than in said second bellows portion.