To enhance the working efficiency of a small pump using diaphragms by facilitating assembling of an eccentric rotary shaft for vertically moving the diaphragms.

In the small pump 23 to perform the pumping operation by vertically moving the diaphragms 4 and 4, a lower end part of the eccentric rotary shaft 13 to vertically move the diaphragms 4 and 4 via an oscillating body 6 is oscillatably fitted with a play to a hole 25 eccentric ally drilled in a rotating body 24, and the rotating body 24 is rotated by a motor 19 to eccentrically rotate the eccentric rotary shaft 13 of the small pump 23. A guide port 26 is formed by chamfering an opening part of a hole 25 drilled in the rotating body 24, a guide groove 27 is formed tapered by inclined to the hole 25 from a predetermined range in the vicinity of a center part of the rotating body 24, and the lower end part of the eccentric rotary shaft 13 is guided into the hole 25 by the guide of the guide groove 27 and the guide port 26 when assembling the eccentric rotary shaft 13.