A mowing machine includes a harvesting mechanism which extends during mowing transversely to a working direction, and a lightening device for the harvesting mechanism which includes at least one elastically deformable member whose action can be adjusted by way of an adjustment device connected to the elastically deformable member. The elastically deformable member can include a deformable envelope which contains a gas under adjustable pressure. The adjustment device can include a supply circuit provided with at least one electrovalve or gate connected to the deformable envelope. In an alternative embodiment, the adjustment device can include at least one jack connected to a mechanical spring, and a supply circuit provided with at least one gate or electrovalve connected to the jack. The elastically deformable member of the present invention permits the harvesting mechanism to quickly adjust to a relief of the terrain in which the mowing machine operates while preserving a certain amount of lightening of the harvesting mechanism during its adjustment.