Rotational force from a driving device 11 is transmitted to an eccentric cam 32 through a rotating rod 31, and the eccentric cam 32 causes a piston 22 and a piston rod 25 to move up and down in a reciprocating manner by its rotation. Atmospheric air is inspired into a first chamber R1 in a cylinder 21 upon the descent of the piston 22. Upon the rise of the piston 22, the air in the first chamber R1 is compressed, and the compressed high-pressure air is discharged to an accumulator 12 through a discharge valve 25. The accumulator communicates with a second chamber R2 in the cylinder 21. When the air pressure in the accumulator 12 increases, this high-pressure air pushes the piston 22 to cancel the contact between the piston rod 26 and the eccentric cam 32, whereby the power transmission from a power transmission device 30 to a pressure conversion mechanism 20 is cut off.