PROBLEM TO BE SOLVED: To provide a suspension device, and a method of machining an outer tube of the suspension device, configured to facilitate machining of the outer tube.SOLUTION: A suspension device has an outer tube 1 which is formed in a tubular shape, and includes thickness changed parts 1a, 1b where thicknesses of left and right parts 12, 13 are formed thinner than thicknesses of front and rear parts 10, 11. When the thickness change parts 1a, 1b are cut in a radial direction, an outer circumference 2 of the thickness changed parts is formed of front, rear, left and right circular arcs 20, 21, 22 and 23. Centers p1 of curvature of front and rear circular arcs are the same. Curvature radii r1, r2 of the front and rear circular arcs 20, 21 are formed to be equal to each other. Centers of curvature of the left and right circular arcs 22, 23 are shifted in a direction separating from the left and right circular arcs 22, 23 with respect to the centers p1 of curvature of the front and rear circular arcs 20, 21. The curvature radii r3, r4 of the left and right circular arcs 22, 23 are formed to be larger than the curvature radii r1, r2 of the front and rear circular arcs 20, 21.