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.