PURPOSE: To make thinning compatible with reducing pulsation caused by multiple cylinder structure by forming this four-cylinder structure, provided with four pump chambers in all, into the same thickness as the thickness of a two- cylinder type.

CONSTITUTION: An air pump is provided with a double-shaft type motor 1, a pair of driving shafts 13, 13, a pair of two-cylinder type pump parts 2, 2. In one pump part 2, the respective diaphragms of two pump chambers are connected to the driving shaft 13 provided at one end of an output shaft 10. In the other pump part 2, the respective diaphragms of two pump chambers are connected to the driving shaft 13 provided at the other end of the output shaft 10. Two pump chambers in each pump part 2 are disposed in the same direction in the orthogonal direction to the axial direction of the motor 1. A pair of driving shafts 13, 13 provided at the output shaft 10 of the motor 1 are shifted approximately by 90° in the decentered direction thereof. As a result, the respective intake-exhaust stroke phases of four pump chambers are all shifted so as to reduce pulsation.