A cylinder device includes a cylinder that has one end from which a piston rod extends, a pipe-shaped cover member that has an extendable bellows portion and a small-diameter portion whose inner diameter is smaller than an inner diameter of the bellows portion, the cover member being configured to protect the piston rod, and a capping member fitted into the cylinder, the capping member having a convex portion to which the small-diameter portion of the cover member is locked, at an outer periphery, wherein at the small-diameter portion of the cover member, a bulge portion that bulges outside in a radial direction is formed.