A hybrid power transmission mechanism comprising: an engine ; a motor/generator having a rotor and a stator ; a driven machine to which power from the engine and/or the motor/generator is to be transmitted; a planetary gear mechanism having a sun gear , an internal gear , a planetary gear configured to mesh with the sun gear and the internal gear , and a planetary carrier shaft configured to support the planetary gear rotatably around the sun gear ; a first clutch ; and a second clutch , wherein the planetary carrier shaft includes a first extension shaft part and a second extension shaft part extending from the planetary gear in opposite directions along a rotational axis direction of the planetary gear , wherein a first shaft configured to output power of the engine is drivingly connected to the first extension shaft part of the planetary carrier shaft , wherein a second shaft configured to input power to the driven machine is drivingly connected to the second extension shaft part of the planetary carrier shaft , wherein the internal gear is drivingly connected to the rotor , wherein the first clutch is switchable between a connected state of allowing transmission of power between the first shaft and the internal gear and a disconnected state of not allowing transmission of power between the first shaft and the internal gear , and wherein the second clutch is switchable between an allowing state of allowing rotation of the sun gear and a blocking state of not allowing rotation of the sun gear , the hybrid power transmission mechanism being characterized in that it further comprises a speed changing mechanism configured to reduce a speed of rotation of the internal gear and to transmit the speed-reduced rotation of the internal gear to the first shaft when the first clutch is in the connected state. an engine ; a motor/generator having a rotor and a stator ; a driven machine to which power from the engine and/or the motor/generator is to be transmitted; a planetary gear mechanism having a sun gear , an internal gear , a planetary gear configured to mesh with the sun gear and the internal gear , and a planetary carrier shaft configured to support the planetary gear rotatably around the sun gear ; a first clutch ; and a second clutch , wherein the planetary carrier shaft includes a first extension shaft part and a second extension shaft part extending from the planetary gear in opposite directions along a rotational axis direction of the planetary gear , wherein a first shaft configured to output power of the engine is drivingly connected to the first extension shaft part of the planetary carrier shaft , wherein a second shaft configured to input power to the driven machine is drivingly connected to the second extension shaft part of the planetary carrier shaft , wherein the internal gear is drivingly connected to the rotor , wherein the first clutch is switchable between a connected state of allowing transmission of power between the first shaft and the internal gear and a disconnected state of not allowing transmission of power between the first shaft and the internal gear , and wherein the second clutch is switchable between an allowing state of allowing rotation of the sun gear and a blocking state of not allowing rotation of the sun gear , the hybrid power transmission mechanism being characterized in that it further comprises a speed changing mechanism configured to reduce a speed of rotation of the internal gear and to transmit the speed-reduced rotation of the internal gear to the first shaft when the first clutch is in the connected state. <EOS>