A closing device for a flap on an incubator hood or thermotherapy apparatus hood is equipped with flaps, which make access possible to the premature or newborn baby from the outside. A closing device for the flap (1) on an thermotherapy apparatus hood (3) is provided which guarantees reliable closing. A roller (9) at a second closing element (5) extends behind a first closing element (4). The roller (9) is mounted on an axle (8) with a clearance in a range of 1.5 mm to 2.0 mm in a direction perpendicular to the direction in which the axle (8) extends, so that an undefined state of the closing device, in which the roller (9), though touching the first closing element (4), does not extend behind it, is avoided.