A hybrid construction machine, comprising: a lower propelling body; an upper frame which is provided so as to be able to slew on the lower propelling body; an engine compartment which is provided in a rear portion of the upper frame; a left and right pair of vertical plates (11, 12) which extend in a front/rear direction in an intermediate portion in a left/right direction of the upper frame; a cabin which is provided in a space above the upper frame at a position on an outside of the both vertical plates (11, 12) in the left/right direction; a work attachment having a base end section which is attached on a front portion of each of the vertical plates (11, 12) by a boom foot pin; a motor-generator capable of operating as an electric motor and an electric generator; an engine serving as a drive source for causing the motor-generator to operate as an electric generator; an electric storage device which constitutes a power source for causing the motor-generator to operate as an electric motor, and which is charged by the motor-generator operating as an electric generator; a slewing electric motor which drives the upper frame to slew by using the electric storage device as a power source; a controller which controls operation of the motor-generator, the slewing electric motor and the electric storage device; and a device case which internally accommodates the electric storage device and the controller, wherein the device case is provided in a space above the upper frame, in a front portion of the upper frame and on an opposite side of the both vertical plates (11, 12) from the cabin, and constitutes up-and-down steps which are formed in a rearward-ascending step shape so as to be able to be ascended and descended, characterized in that the device case includes a cooling water inlet for introducing cooling water for cooling the electric storage device and the controller inside the device case, and a cooling water outlet for extracting the cooling water, the cooling water inlet and the cooling water outlet being provided on a side surface of the device case which faces an adjacent-side vertical plate, of the two vertical plates (11, 12), which is on a side adjacent to the device case, at positions distanced from the adjacent-side vertical plate when viewed from the side. a lower propelling body; an upper frame which is provided so as to be able to slew on the lower propelling body; an engine compartment which is provided in a rear portion of the upper frame; a left and right pair of vertical plates (11, 12) which extend in a front/rear direction in an intermediate portion in a left/right direction of the upper frame; a cabin which is provided in a space above the upper frame at a position on an outside of the both vertical plates (11, 12) in the left/right direction; a work attachment having a base end section which is attached on a front portion of each of the vertical plates (11, 12) by a boom foot pin; a motor-generator capable of operating as an electric motor and an electric generator; an engine serving as a drive source for causing the motor-generator to operate as an electric generator; an electric storage device which constitutes a power source for causing the motor-generator to operate as an electric motor, and which is charged by the motor-generator operating as an electric generator; a slewing electric motor which drives the upper frame to slew by using the electric storage device as a power source; a controller which controls operation of the motor-generator, the slewing electric motor and the electric storage device; and a device case which internally accommodates the electric storage device and the controller, wherein the device case is provided in a space above the upper frame, in a front portion of the upper frame and on an opposite side of the both vertical plates (11, 12) from the cabin, and constitutes up-and-down steps which are formed in a rearward-ascending step shape so as to be able to be ascended and descended, characterized in that the device case includes a cooling water inlet for introducing cooling water for cooling the electric storage device and the controller inside the device case, and a cooling water outlet for extracting the cooling water, the cooling water inlet and the cooling water outlet being provided on a side surface of the device case which faces an adjacent-side vertical plate, of the two vertical plates (11, 12), which is on a side adjacent to the device case, at positions distanced from the adjacent-side vertical plate when viewed from the side.