a power transmission device is provided , the power transmission device comprising : a motor including a rotor having a permanent magnet embedded therein and a stator , and configured to generate a rotational force ; a reducer configured to transmit the rotational force of the motor ; and a wet brake mechanism configured to put a brake the rotation of the motor . spaces accommodating the motor , the reducer , and the brake mechanism communicate with each other so that lubricant oil can flow through the spaces . a leakage magnetic flux at an axial end face of the rotor away from the brake mechanism is greater than that of the end face toward the brake mechanism .