To make discharging to the outside of a cylinder unit possible of only air from in air mixed oil in an oil chamber in a cylinder unit under a bush held to a bearing, prevent the air mixed oil from being left in the cylinder unit.

In an internal peripheral surface of a rod unit insertion hole formed in a shaft core part of a bearing mounted in an opening end of a cylinder unit, a peripheral surface of a rod unit protruding/sinking relating to the cylinder unit holding this peripheral surface in an adjacent condition is brought into slide contact by a bush 1. In a bearing part structure formed by having the bush 1, an air bleed groove 1a connecting an oil chamber in the cylinder unit under this bush 1 to an oil reservoir chamber R2 in the upward of this bush 1 is formed in a peripheral surface of the bush 1, this air bleed groove 1a, which a high pressure is generated in the cylinder unit, while impeding passing of oil to the oil reservoir chamber R2, permits passing of air, and at negative pressure time in the cylinder unit, impedes suction of air from the oil reservoir chamber R2, in a trap effect, so as to be able to display it, and the groove is set.