PURPOSE: To obtain a small-size and large capacity pump whose inner coil is efficiently cooled by a method wherein a heat-pipe is provided at the center part in an inner duct and the radiation part of the heat-pipe is provided on the exit side of an annulus flow path to discharge heat generated in the inner coil efficiently.

CONSTITUTION: Heat generated in an inner coil 7 is discharged through an inner stator core 8, an inner duct 6 and fluid and, at the same time, reaches the operating liquid in a heat-pipe 12 through the inner stator core 8 and the heat-pipe 12 to vaporize the liquid. The vaporized working liquid reaches a radiation part 12a outside the inner duct and is liquefied by cooling fluid. Thus the heat is discharged. As the heat generated in the inner coil 7 which is a heat generating part can be efficiently transmitted from the inner duct to liquid sodium, so that the temperature rise of the inner coil 7 can be allowed.