Dual clutch transmission for vehicle

A dual clutch transmission for a vehicle, the dual clutch transmission comprising:

• a first input shaft to which a first clutch is connected;
• a second input shaft to which a second clutch is connected, the second input shaft being mounted coaxially with the first input shaft;
• a first output shaft, a second output shaft and a third output shaft mounted in parallel with the first input shaft and the second input shaft;
• and
• a combined gear group including first and second pairs of external gears configured to be connected in series to each other or disconnected from each other so that the first input shaft is decelerated in at least two steps using the first output shaft to enable the second input shaft to be driven in a direction in which the first input shaft is driven.