An installation structure which is installable to an inner periphery of a shaft hole provided in a housing, comprising: a lip type seal slidably in close contact with a peripheral surface of a shaft inserted to said shaft hole; and a cartridge arranged at an outer peripheral side of the lip type seal; wherein the installation structure is made such that said lip type seal is installable to said inner periphery of the shaft hole via the cartridge, said cartridge having a small-diameter tube portion, an annular step portion integrally formed on a sealed fluid side end portion of the small-diameter tube portion toward an outer side in a radial direction, and a large-diameter tube portion integrally formed on an outer peripheral end portion of the step portion toward the sealed fluid side A; and said lip type seal comprises a first lip seal member and a retaining ring, said first lip seal member integrally having a cylinder portion, and annular radial portion integrally formed from an end portion at a sealed fluid side of the cylinder portion toward an inner side in a radial direction, and a first seal lip integrally formed on an inner peripheral end portion of the annular radial portion so as to extend toward the sealed fluid side, said retaining ring comprising a tubular portion formed integrally with the retaining ring, characterized in that said lip type seal further comprises a second lip seal member, a backup ring, said second lip seal member comprises an annular flat surface portion, and a second seal lip integrally formed on an inner peripheral end portion of the annular flat surface portion in such a manner as to slope to the sealed fluid side, said backup ring is formed in an annular flat shape, and arranged further in an atmospheric air side than the second lip seal member to carry out a backing up operation with respect to the second lip seal member, said retaining ring further comprises a fold-back portion provided continuously to a sealed fluid side end portion of the tubular portion, an inward flange portion provided continuously to the fold-back portion, and a caulking portion provided on the atmospheric air side end portion of the tubular portion, wherein the second lip seal member and the backup ring are caulked and fixed between the inward flange portion and the caulking portion, wherein said annular step portion of the cartridge is formed in an S-shaped cross sectional shape, or an approximately S-shaped cross sectional shape, so that the small-diameter tube portion and the large-diameter tube portion are arranged so as to partly overlap in the axial direction, and said cartridge is structured such as to be fittable to the inner peripheral surface of the shaft hole by an outer peripheral surface of the large-diameter tube portion, while the lip type seal is fitted to an inner peripheral surface of the small-diameter tube portion. a lip type seal slidably in close contact with a peripheral surface of a shaft inserted to said shaft hole; and a cartridge arranged at an outer peripheral side of the lip type seal; wherein the installation structure is made such that said lip type seal is installable to said inner periphery of the shaft hole via the cartridge, said cartridge having a small-diameter tube portion, an annular step portion integrally formed on a sealed fluid side end portion of the small-diameter tube portion toward an outer side in a radial direction, and a large-diameter tube portion integrally formed on an outer peripheral end portion of the step portion toward the sealed fluid side A; and said lip type seal comprises a first lip seal member and a retaining ring, said first lip seal member integrally having a cylinder portion, and annular radial portion integrally formed from an end portion at a sealed fluid side of the cylinder portion toward an inner side in a radial direction, and a first seal lip integrally formed on an inner peripheral end portion of the annular radial portion so as to extend toward the sealed fluid side, said retaining ring comprising a tubular portion formed integrally with the retaining ring, characterized in that said lip type seal further comprises a second lip seal member, a backup ring, said second lip seal member comprises an annular flat surface portion, and a second seal lip integrally formed on an inner peripheral end portion of the annular flat surface portion in such a manner as to slope to the sealed fluid side, said backup ring is formed in an annular flat shape, and arranged further in an atmospheric air side than the second lip seal member to carry out a backing up operation with respect to the second lip seal member, said retaining ring further comprises a fold-back portion provided continuously to a sealed fluid side end portion of the tubular portion, an inward flange portion provided continuously to the fold-back portion, and a caulking portion provided on the atmospheric air side end portion of the tubular portion, wherein the second lip seal member and the backup ring are caulked and fixed between the inward flange portion and the caulking portion, wherein said annular step portion of the cartridge is formed in an S-shaped cross sectional shape, or an approximately S-shaped cross sectional shape, so that the small-diameter tube portion and the large-diameter tube portion are arranged so as to partly overlap in the axial direction, and said cartridge is structured such as to be fittable to the inner peripheral surface of the shaft hole by an outer peripheral surface of the large-diameter tube portion, while the lip type seal is fitted to an inner peripheral surface of the small-diameter tube portion.