The invention relates to a propulsion unit (1) for an aircraft comprising a nacelle (2) comprising a turbojet (8) supported by a pylon (3), the nacelle (2) comprising: - an external fairing (9), - a structure fixed internal (4) delimiting with the external fairing (9) an annular space (6) in which is able to circulate a stream of cold air (7), and - a seal (10) positioned the fixed internal structure (4) and the pylon (3), said seal (10) being intended to separate a cold pylon zone (11) from a hot engine zone (12). According to the invention, the propulsion unit (1) comprises a ventilation device (13) making it possible to deflect a portion of cold air (14) from the cold air stream (7) to the seal ( 10) to ventilate it.
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