The invention relates to a device for accumulating inertial energy, comprising a housing (12) defining a rotor chamber (16), at least one rotor (18) having a bottom end face (26) and a substantially opposite top end face (24), the rotor (18) being mounted in the rotor chamber (16) so that it can rotate relative to the housing (12) about a vertical axis of rotation (20) leaving free a gap (32), wherein a seal (36) is provided in the gap (32) separating an upper section (34) of the rotor chamber (16) and a bottom section (38) of the rotor chamber (16), a means (40) for exposing at least the bottom end face (26) of the rotor (18) in the bottom section (38) to a gas pressure which, by comparison with the pressure applied to the substantially opposite top end face (24) in the upper section (34) generates an upward differential pressure force that at least partially compensates for the weight of the rotor (18), and the device is provided with means (40) for reducing the pressure in said upper section (18).