An inverter/converter as a kind of hybrid unit is composed of a body and a casing covering the body and is disposed in an engine room. An air intake port and an air discharge port each provided with a fan are formed in the casing to configure an internal air cooling device for introducing air present outside the engine room into the casing, allowing the air to flow through the interior of the casing and then discharging it from the air discharge port. A power divider for distributing engine power to a hydraulic pump and a generator-motor is disposed on one of right and left sides of an enginein a rear portion of an upper rotating body. Further, an electric power storage device and the inverter/converter are disposed right and left in parallel behind a cabin and in front of the engine, and a partition wall which defines the engine room is configured by casings of the electric power storage device, the inverter/converter and the power divider.