To improve the durability of a dust cover, by preventing the generation of a bending to the duct cover, when abutting to a spring seat.

An outer casing 1 whose one end side 1a is sealed; a piston rod 2 extending by penetrating the other end side 1b of the outer casing 1; a dust cover 3 whose upper end 3a is connected to the piston rod 2, to cover the outer periphery of the outer casing 1; and a spring seat 4 installed to the outer periphery of the outer casing 1, and to support a suspension spring 6; are provided. To the spring seat 4, plural projections 9 are formed by cutting up the parts of the spring seat 4, and they are arranged in a circular form to surround the outer casing 1. The lower end 3b of the dust cover 3 can be supported by the projections 9 arranged in a circular form.