To suppress a generation of electroerosion of a magnesium component in a bearing device, which is formed by a magnesium-based component consisting of magnesium-based material coated by insulator, for supporting a rotary shaft consisting of a different kind of material from the magnesium-based material.

The bearing device is characterized in that a hollow 25 is formed in a seat rail 21L of rear frame consisting of magnesium alloy, in that a bearing member 31 consisting of synthetic resin is fitted into the hollow 25, an iron rotary shaft 51 is rotatably supported in a hollow 32 of the bearing member 31, and a press member 33 made of aluminum alloy is fixed with screw in the seat rail 21L in such a state that the rotary shaft 51 is pressed down at bearing member 31 side.