The steam generator according to the invention consists of a main vessel 1 equipped with heating devices 12 controlled by control devices 13. The water level in the steam generator is kept at an average level 5 by control devices 13 which control a water inlet solenoid valve 10. Measurement of the average water level 5 is achieved by a pressure switch 20 that is exposed to the pressure inside a measuring vessel 18 that communicates with the main vessel 1 via a lower measuring opening 22. Balancing pipework 34 links the pressure switch 20 to the pressure of the steam produced in the steam generator. Pressure switch 20 generates measuring signals that are sent via conductors 17 to control devices 13 in order to actuate solenoid valve 10.