Transmission

1. A transmission comprising:

a case;

a plurality of gears rotatably supported in the case;

lubrication oil contained in the case;

a lubrication oil storage region in a bottom portion of the case, storing the lubrication oil, and enabling the stored lubrication oil to be scooped up by rotation of a first gear of the plurality of gears above the first gear;

a receiver for flowing the lubrication oil scooped up upward by the rotation of the first gear to sliding parts of the gears; and

a lubrication oil retention section having a U-letter shape that surrounds a circumferential surface and both side surfaces of a circumferential part of the first gear, the lubrication oil retention section extending from the lubrication oil storage region to an outside of the lubrication oil storage region at a portion on the circumferential part where the lubrication oil is splashed towards the receiver, and the lubrication oil retention section retaining the lubrication oil scooped up by the rotation of the first gear between itself and the first gear at the outside of the lubrication oil storage region and enabling the first gear to splash toward the receiver the lubrication oil retained at the outside of the lubrication oil storage region.

2. The transmission in claim 1 , wherein the lubrication oil retention section includes falling prevention portions that retain on side surfaces of the first gear the lubrication oil that is retained by the lubrication oil retention section and that has been scooped up by the rotation of the first gear, the falling prevention portions being bodily joined or formed along upper portions of the side surfaces of the first gear that are above an axis center of the first gear.

3. The transmission in claim 1 , wherein the U-letter shape of the lubrication oil retention section below the axis center of the first gear spreads outward.

4. The transmission in claim 1 , wherein:

the lubrication oil retention section includes two side members respectively secured to first and second case sections that constitute the case, and

the two side members surround the circumferential surface and the both side surfaces of the circumferential part of the first gear with the second case section fastened to the first case section.

5. The transmission in claim 1 , wherein the first gear comprises a gear of the plurality of gears that is rotated at all times during a vehicle travelling.

6. The transmission in claim 5 , wherein the transmission is an automatic dual-clutch transmission that further comprises:

a first input shaft and a second input shaft arranged concentrically and rotatably supporting gears on a drive side of the plurality of gears;

a first output shaft and a second output shaft arranged in parallel to the first input shaft and rotatably supporting gears on a driven side of the plurality of gears; and

a dual clutch having a first clutch for transmitting a rotational driving power of a prime mover to the first input shaft and a second clutch for transmitting the rotational driving power to the second input shaft;

wherein the first gear is a ring gear of a differential gear that is rotationally connected to the first and second output shafts at all times.

7. The transmission in claim 2 , wherein the falling prevention portions extend in toward the respective side surfaces of the first gear.

8. The transmission in claim 1 , wherein respective distances from the lubrication oil retention section to the circumferential surface and to the both side surfaces of the circumferential part of the first gear are kept constant.

9. The transmission in claim 2 , wherein the falling prevention portions are formed at an open end of the U-letter shape and respectively bend inwardly to retain the lubrication oil scooped up by the first gear therein.