A pneumatic cylinder has a shaft member having a piston and a piston rod combined with each other in an axial direction, a cylinder body supporting the shaft member with the piston rod protruding outwardly therefrom so as to freely reciprocate linearly, and porous air bearings and which are incorporated into the cylinder body and slidably support the piston and the piston rod. Materials whose thermal expansion coefficients are approximately equivalent are used for the shaft member, the cylinder body, and the air bearings. While gaps between the shaft member and the air bearings are held constant, and the functions of air layers formed in the gap, namely, a sliding function and a sealing function can be maintained stably regardless of temperature change.