An air cylinder that changes the posture of an automatic wet sanding unit main body having sandpaper mounted thereon is provided with a guide rod, and an outer circumferential surface of the guide rod has grooves that extend along a shaft centerline of the guide rod and have an arc-shaped cross-section. Balls are interposed between a bottom of each groove and an inner surface of a bush that is provided inside the air cylinder. Thus, it is possible to make two objects compatible with each other: to achieve high-accuracy automatic wet sanding by enhancing the adaptability of the sandpaper to the shape of a painted surface through a reduction of the diameter of the piston rod; and to enhance the durability of the automatic wet sanding apparatus.