To effectively cool a bearing inside a piston.

This air compressor 10 that is an oilless type (an unlubricated type) reciprocation compressor has a piston 11 and a cylinder 12. A crankshaft is installed with a vane plate 25 for generating an airflow cooling the inside of the piston 11 in a crank chamber. The vane plate 25 is installed on the periphery of the crankshaft, such that the vane plate 25 extends parallel to the radial direction, while a front end 25a of the vane plate 25 rotates with a phase difference of 180 degrees to reciprocation of the piston 11. When the piston 11 moves to the bottom dead center, the vane plate 25 installed on the periphery of the crankshaft rotates together with the rotation of the crankshaft and presses air in the crank chamber in the rotating direction to generate airflow. Accordingly, when the piston 11 moves down to the bottom dead center, cooling air is supplied inside the piston 11 to further improve cooling efficiency.