PURPOSE: To obtain the stable linear operation and improve the durability in a captioned pump by connecting a connection rod with a piston guided and held to a cylinder and the piston with an elastic operation member such as a bellows and the like.

CONSTITUTION: When an electric motor 1 is operated and an eccentric rotation body 11 is given force by a shaft 12, a piston 16 is vertically reciprocated via a connection rod 13b. When the piston 16 is reciprocated, a cap 24 fixed at its top is vertically reciprocated, and a bellows 24 is vertically expanded and contracted as a whole as interlocking with the cap 24. In this air pump, only linear reciprocation is taken out from the reciprocation of the rod 13b by a cylinder 15 and the piston 16 and transmitted to the bellows 26. Consequently, the beloows 26 is prevented from oscillating and free from the addition of unnecessary and unnatural force as expanding and contracting linearly.