PURPOSE: To perform smooth slide through the medium of a lubricant for a long period by a method wherein the peripheral end part on the air compression side of a piston body is formed in an edge-form shape and an annular recessed part for a lubricant is formed in the outer periphery of the piston body.

CONSTITUTION: A piston body 4 of a resilient substance having the conical large part side installed on the compression chamber 12 side of a cylinder 1 is provided, and the peripheral end part on the air compression chamber 12 side of the piston body 4 is formed in an edge-form shape. An annular recessed part 7 for a lubricant is formed in the outer periphery of the piston body 4. Thus, the outer periphery of the piston body 4 has the recessed part 7 stored with a lubricant, such as grease 11. The piston body 4 is assembled such that a piston rod 2 is inserted in a distance pipe 8 and fixed through washers 9 and 9 on both sides of the piston body by means of a nut 10, and the piston body is inserted in the cylinder 1 in a state to fill the recessed part 7 with the grease 11. Since, in this constitution, the recessed part 7 is allowed to store the lubricant 11, an air pump can be operated without supplying the lubricant 11 for a long period.