A power transmission unit for a vehicle (Ve), comprising: a prime mover including a main prime mover (1, 32), a first motor and a second motor; a transmission configured to transmit torque between the main prime mover (1, 32) and drive wheels and between the first motor and the drive wheels; a locking device (23, 26) configured to selectively halt a rotation of an output shaft of the main prime mover (1, 32) by bringing a rotary member (23b, 26b) into engagement with a stationary member (23a, 26a); a transmission case which holds the first motor, the second motor, the transmission, and the locking device (23, 26) and a first torque limiter that is disposed between the main prime mover (1, 32) and the locking device (23, 26) to restrict torque transmitted between the drive wheels and the main prime mover (1, 32), characterized in that the locking device includes a dog clutch that comprises the stationary member and the rotary member and is held in the transmission case, wherein the stationary member is fixed to the transmission case while being allowed to move in a direction of a rotational axis of the rotary member but not allowed to rotate, the stationary member being pushed toward the rotary member by a predetermined pushing force (F 1), wherein the locking device is further configured to: establish a component (F 2) of a torque in the direction of the rotational axis of the rotary member by applying the torque to a contact site between engagement faces of the stationary member and the rotary member; maintain the engagement between the stationary member and the rotary member if the component (F 2) of the torque applied to the rotary member is weaker than the pushing force (Fi), when the stationary member is brought into engagement with the rotary member; and serve as a second torque limiter (24, 27) to restrict torque transmitted between the drive wheels and the locking device by disengaging the stationary member from the rotary member if the component (F 2) of the torque applied to the rotary member is greater than the pushing force (F 1) when the stationary member is brought into engagement with the rotary member. a prime mover including a main prime mover (1, 32), a first motor and a second motor; a transmission configured to transmit torque between the main prime mover (1, 32) and drive wheels and between the first motor and the drive wheels; a locking device (23, 26) configured to selectively halt a rotation of an output shaft of the main prime mover (1, 32) by bringing a rotary member (23b, 26b) into engagement with a stationary member (23a, 26a); a transmission case which holds the first motor, the second motor, the transmission, and the locking device (23, 26) and a first torque limiter that is disposed between the main prime mover (1, 32) and the locking device (23, 26) to restrict torque transmitted between the drive wheels and the main prime mover (1, 32), characterized in that the locking device includes a dog clutch that comprises the stationary member and the rotary member and is held in the transmission case, wherein the stationary member is fixed to the transmission case while being allowed to move in a direction of a rotational axis of the rotary member but not allowed to rotate, the stationary member being pushed toward the rotary member by a predetermined pushing force (F 1), wherein the locking device is further configured to: establish a component (F 2) of a torque in the direction of the rotational axis of the rotary member by applying the torque to a contact site between engagement faces of the stationary member and the rotary member; maintain the engagement between the stationary member and the rotary member if the component (F 2) of the torque applied to the rotary member is weaker than the pushing force (Fi), when the stationary member is brought into engagement with the rotary member; and serve as a second torque limiter (24, 27) to restrict torque transmitted between the drive wheels and the locking device by disengaging the stationary member from the rotary member if the component (F 2) of the torque applied to the rotary member is greater than the pushing force (F 1) when the stationary member is brought into engagement with the rotary member. establish a component (F 2) of a torque in the direction of the rotational axis of the rotary member by applying the torque to a contact site between engagement faces of the stationary member and the rotary member; maintain the engagement between the stationary member and the rotary member if the component (F 2) of the torque applied to the rotary member is weaker than the pushing force (Fi), when the stationary member is brought into engagement with the rotary member; and serve as a second torque limiter (24, 27) to restrict torque transmitted between the drive wheels and the locking device by disengaging the stationary member from the rotary member if the component (F 2) of the torque applied to the rotary member is greater than the pushing force (F 1) when the stationary member is brought into engagement with the rotary member.