PURPOSE: To reduce the weight of an air cylinder and to secure steady performance and durability by forming the sliding face of an air cylinder body of a chilled crystall layer which is few in very fine casting defect occuring in casting an aluminum alloy.

CONSTITUTION: An air cylinder body 1 is coupled with an oil cylinder 2 to form a pressure intensifier. A piston 3 is fitted in the air cylinder 1, and a piston rod 4 coupled with the piston 3 forms a plunger fitted in the cylinder part 5 of the oil cylinder body 2. A packing 6 and a guide ring 7 are attached to the piston 3, and brought into sliding contact with the sliding face 8 of the air cylinder body 1. In this case, the air cylinder body 1 is molded by aluminum alloy casting, and its sliding face 8 is finished by turning and honing processes. Many chillers are set in a sand mold opposite to the sliding face 8 in order to thickly produce fine chilled crystals on the sliding face 8.