A closure device for a fluid container, in particular a hydraulic tank, has a base (4), which can be attached to the container opening and has a cover part (2), which encloses a ventilation and/or a filter device (36). The cover part (2) is held on the base (4) so as to be freely rotatable in a functional unlocking position. In a functional locking position, the closure device permits the container opening to be unlocked by a safety device (48, 50, 58) while connecting the cover part (2) to the base (4). The safety device (48, 50, 58) can be actuated by a magnetic device generating a magnetic field (62). The magnetic device establishes the functional locking position of the cover (2) and the base (4) by magnetic force.