Air distribution device for a vehicle

The device has a rotary unit (2) mounted in a case to take two angular positions. A median air passage (F1) is freed and two lateral air passages are closed in one of the positions. The passage (F1) is closed and the lateral air passages are freed in another position. Openings in transversal passages permit additional air flow to rejoin the passage (F1) from top part of the lateral air passages.