A safety helmet with a helmet shell, with a bearing ring, with a rotatable actuating element (14) and with a transmission unit (15.1, 15.2). The transmission unit (15.1, 15.2) transmits a rotation of the actuating element to the bearing ring, so that the head size provided is changed. A helmet shell-side transmission piece (15.1) is non-rotatably connected to the actuating element (14) such that they rotate in unison, and a bearing ring-side transmission piece (15.2) is connected mechanically to the bearing ring. The helmet shell-side transmission piece (15.1) can move both relative to the actuating element (14) and relative to the bearing ring-side transmission piece (15.2) linearly in two opposite directions. The distance between the actuating element (14) and the bearing ring can be changed thereby.