A sealing unit with a mounting error preventing construction, includes a cylindrical member and an annular sealing member mounted in a circumferential groove formed on an outer circumference of the cylindrical member. At least two projections having different sizes are provided on the sealing member asymmetrically with respect to an axis of the sealing member and are projected in a direction in which the axis extends. Cutout portions for receiving the projections are provided on the cylindrical member.