To solve the following problem with a conventional coolant pump: a boundary friction state is made to damage a bearing surface because of the use of an expensive bush in a bearing which does not have such a structure as to positively flow coolant to between a driving shaft and the bearing.

A groove 26 is structured in a communication hole 18, an annular groove 17 and a cylinder bearing 8 in the driving shaft 11 so that coolant may be supplied to the bearing portion. With this constitution, the coolant is positively supplied to the bearing portion, thus realizing a profitable plain bearing against wear.