PURPOSE: To prevent seizure between a bearing and a shaft due to their swelling by changing the inside diameter of the bearing with no uniformity in its longitudinal direction, and decreasing the area of a part which has a regular gap at the initial time of water immersion to reduce increase in frictional torque.

CONSTITUTION: Both the ends of a bearing material 1 such as rubber and phenol fitted in a back shell 2 are made a diameter which is a regular gap at the initial time of water immersion, and its central part is made a diameter where the regular gap + secular swelling amount are taken into consideration. It is thus possible to prevent the shaft from being stuck due to swelling and frictional torque from increasing, providing high reliability.