A hybrid power transmission mechanism includes an engine; a motor/generator having a rotor and a stator; a driven machine; a planetary gear mechanism having a sun gear, an internal gear drivingly connected to the rotor, a planetary gear, and a planetary carrier shaft including first and second extension shaft parts extending from the planetary gear in opposite directions; a first clutch switchable between a state of allowing transmission of power between the first shaft and the internal gear and a state of not allowing transmission of power therebetween; and a second clutch switchable between a state of allowing rotation of the sun gear and a state of not allowing rotation thereof. A first shaft that outputs power of the engine and a second shaft that inputs power to the driven machine are drivingly connected to the first and second extension shaft parts of the planetary carrier shaft, respectively.