An upper slewing body for a hybrid construction machine, which is provided rotatably on a lower propelling body, comprising: an upper frame; a first vertical plate and a second vertical plate forming a left-right pair and having conductivity, which stand on the upper frame in a left-right direction intermediate portion of the upper frame with a left-right direction interval, and extend over substantially an entire front-rear direction length of the upper frame; an engine provided in a rear portion of the upper frame; a generator motor that is provided in the rear portion of the upper frame and can be operated as a generator using power from the engine; an electric storage device that is provided in a front portion of the upper frame and constitutes a power source for operating the generator motor as a motor; a control device provided in the front portion of the upper frame to control operations of the electric storage device and the generator motor; an electric power cable that connects the electric storage device to the generator motor in order to transmit electric power therebetween; and a signal cable that connects the control device to the generator motor in order to transmit signals therebetween, characterized in that the generator motor, the electric storage device, and the control device are disposed in an outside space on an opposite side of the first vertical plate to the second vertical plate, a front portion cable insertion hole is provided in a front portion of the first vertical plate and a rear portion cable insertion hole is provided in a rear portion of the first vertical plate, and one of the electric power cable and the signal cable is an outside cable laid along a route that passes through only the outside space in a lower position than an upper end of the first vertical plate, while the other cable is a bypass cable laid along a bypass route that extends from the generator motor in a lower position than the upper end of the first vertical plate, passes through the rear portion cable insertion hole into an intermediate space between the first vertical plate and the second vertical plate, and returns to the outside space from the intermediate space through the front portion cable insertion hole. an upper frame; a first vertical plate and a second vertical plate forming a left-right pair and having conductivity, which stand on the upper frame in a left-right direction intermediate portion of the upper frame with a left-right direction interval, and extend over substantially an entire front-rear direction length of the upper frame; an engine provided in a rear portion of the upper frame; a generator motor that is provided in the rear portion of the upper frame and can be operated as a generator using power from the engine; an electric storage device that is provided in a front portion of the upper frame and constitutes a power source for operating the generator motor as a motor; a control device provided in the front portion of the upper frame to control operations of the electric storage device and the generator motor; an electric power cable that connects the electric storage device to the generator motor in order to transmit electric power therebetween; and a signal cable that connects the control device to the generator motor in order to transmit signals therebetween, characterized in that the generator motor, the electric storage device, and the control device are disposed in an outside space on an opposite side of the first vertical plate to the second vertical plate, a front portion cable insertion hole is provided in a front portion of the first vertical plate and a rear portion cable insertion hole is provided in a rear portion of the first vertical plate, and one of the electric power cable and the signal cable is an outside cable laid along a route that passes through only the outside space in a lower position than an upper end of the first vertical plate, while the other cable is a bypass cable laid along a bypass route that extends from the generator motor in a lower position than the upper end of the first vertical plate, passes through the rear portion cable insertion hole into an intermediate space between the first vertical plate and the second vertical plate, and returns to the outside space from the intermediate space through the front portion cable insertion hole.