To improve pump efficiency by suppressing reverse flow of fluid in a pump chamber.

This pump driving device is provided with a guide cylinder 23 disposed in the pump chamber 8 coaxially with an output shaft 4 of a motor M and fitted to and sliding with a case inner wall surface 24 having axial both sides forming the pump chamber 8 and an axis side inner wall surface 25 of an impeller 9 housed in the pump chamber 8.