A submersible motor pump, comprising: a water jacket having a circulation passage of a coolant; a motor surrounded by said water jacket; a rotational shaft rotated by said motor; a main impeller secured to said rotational shaft; a centrifugal impeller for circulating the coolant, said centrifugal impeller being rotatable together with said rotational shaft; a suction passage configured to provide fluid communication between said circulation passage and a fluid inlet of said centrifugal impeller; a discharge passage (63, 80) configured to provide fluid communication between a fluid outlet of said centrifugal impeller and said circulation passage, and an annular wall having a horizontal extension wall located above said main impeller; wherein said discharge passage (63, 80) includes a heat-exchange passage formed by two wall surfaces facing each other, wherein one of said two wall surfaces is constituted by said annular wall which contacts a liquid conveyed by said main impeller, wherein said heat-exchange passage has a circular shape extending radially outwardly from said fluid outlet of said centrifugal impeller, wherein said heat-exchange passage includes at least one axial passage section (82, 84) having a length component in an axial direction of said rotational shaft, wherein said main impeller has main blades for pressurizing the liquid and rear vanes facing said horizontal extension wall, wherein said annular wall is shaped so as to separate a space above said main impeller into an inner circumferential space and an outer circumferential space, and wherein said horizontal extension wall has a through-hole configured so that part of the liquid conveyed radially outwardly by said rear vanes is returned to said inner circumferential space. a water jacket having a circulation passage of a coolant; a motor surrounded by said water jacket; a rotational shaft rotated by said motor; a main impeller secured to said rotational shaft; a centrifugal impeller for circulating the coolant, said centrifugal impeller being rotatable together with said rotational shaft; a suction passage configured to provide fluid communication between said circulation passage and a fluid inlet of said centrifugal impeller; a discharge passage (63, 80) configured to provide fluid communication between a fluid outlet of said centrifugal impeller and said circulation passage, and an annular wall having a horizontal extension wall located above said main impeller; wherein said discharge passage (63, 80) includes a heat-exchange passage formed by two wall surfaces facing each other, wherein one of said two wall surfaces is constituted by said annular wall which contacts a liquid conveyed by said main impeller, wherein said heat-exchange passage has a circular shape extending radially outwardly from said fluid outlet of said centrifugal impeller, wherein said heat-exchange passage includes at least one axial passage section (82, 84) having a length component in an axial direction of said rotational shaft, wherein said main impeller has main blades for pressurizing the liquid and rear vanes facing said horizontal extension wall, wherein said annular wall is shaped so as to separate a space above said main impeller into an inner circumferential space and an outer circumferential space, and wherein said horizontal extension wall has a through-hole configured so that part of the liquid conveyed radially outwardly by said rear vanes is returned to said inner circumferential space.