To properly maintain the sealing performance of a floating seal by preventing a foreign object from intruding into the floating seal through a gap between a fixed side housing and a rotary side housing.

In the radial direction inner side of the gap 22 formed between a counter face 12E of a seal applied part 12D provided on the fixed side housing 12 and a counter face 15B of a seal applied part 15A provided on the flange 15 of the rotary side housing 14, an annular block ring 39 is provided to block this gap 22. By this, the foreign object such as earth, sand and the like is prevented by the block ring 39 from intruding into the floating seal 34 through the gap 22 so that the sealing performance of the floating seal 34 can properly be maintained. As a result, a sufficient lubricating oil L can be secured inside the rotary side housing 14, and the rotary side housing can be smoothly rotated in the long run.