A ventilation unit (1A) includes: a ventilation member (2) configured to be attached to an opening (11) of a housing (10), the ventilation member (2) including a water-proof gas-permeable membrane (4) for covering the opening (11); a sealing member (6) configured to seal a gap between the housing (10) and the ventilation member (2); and an annular partition wall member (7A) defining an inner space in which the ventilation member (2) is fitted. The partition wall member (7A) is configured to keep in contact with the housing (10), and blocks a clearance formed outside the sealing member (6) and between a peripheral portion of the ventilation member (2) and the housing (10) when the ventilation member (2) is attached to the opening (11) of the housing (10).