PURPOSE: To assure the smooth turning of a mate member in relation to a boss member for a long time by arranging a seal member with simple mounting structure in the sliding section of the boss member and the mate member.

CONSTITUTION: The external bushing 16 fittedly inserted into the bushing fitting hole 13 of an arm boss 11 and the internal bushing 17 into which a connection pin 18 is inserted are slidden on each other, causing each of brackets 15 to turn in relation to the arm boss 11. At this time, the collar section 17C of the internal bushing 17 slidably abuts on the external end face 16A of the external bushing 16A so as to receive an axial load, and a seal member 21 is arranged in the seal containing space P formed between the internal peripheral surface of the bushing fitting hole 13 and the external peripheral surface of the collar section 17C. This constitution surely prevents earth and sand from going between the sliding surfaces of the external bushing 16 and the internal bushing 17, reducing wearing.