PURPOSE: To aim at compactification in a pump part, by forming an outer casing in connecting a first barrel body, a discharge casing, a second barrel body and a suction casing altogether, while installing a flow passage in position between the outer casing and an inner casing.

CONSTITUTION: In this canned motor pump consisting of a barrel type multistage pump 22 and a canned motor 23, an outer casing 30 is constituted of a first body 25, a discharge casing 32, a second barrel body 27 and a suction casing 28, and a flow passage 39 is installed in position between the outer casing and an inner casing 32. Therefore, a difference between internal pressure acting on the outer casing and external pressure acting on the inner casing is made yet more reducible than discharge pressure so that each thickness in these casings is able to be yet thinner, thus compactification is promotable.