PURPOSE: To prevent the intrusion of foreign substances and lessen friction torque with a simple structure by pressing and urging the tip of a seal member toward a tapered surface to make slidable contact therewith when the outside pressure of a support member is higher than the inside one.

CONSTITUTION: When pressure on the outside OS of a support member 16 is higher than that on the inside IS, the tip 23b of a seal member 23 is pressed toward a tapered surface 20, and constitutionally directed to be extruded through the tapered surface 20 to the outside OS of the support member 16 when a rotary shaft 18 is inserted into the support member 16. Thus the rotary shaft 18 is supported by a bearing 19 of the support member 16, while the tip 23b of the seal member 23 makes slidable contact with the tapered surface 20. Then, the tip 23b is urged from the tapered surface 20 to the outside OS of the support member 16. Also, since the seal member 23 makes slidable contact with the tapered surface 20 with small bending, friction torque to hinder the rotation of the rotary shaft 18 can be reduced.