A breathing gas humidifier system for a patient includes a replaceable, closed water reservoir (1) with stable pressure, provided for receiving a water reserve for the breathing gas humidifier system. An intermediate storage unit (4) with water is closed from the environment and is located under the water reservoir. The water reservoir (1) and the intermediate storage unit (4) have a valve (16, 14) each, with one of the valves cooperating with the other valve, so that there is a flow connection between the water reservoir (1) and the intermediate storage unit (4) only when the water reservoir (1) is received in a connector (2) of the intermediate storage unit (4). The intermediate storage unit (4) is connected via a water connection line (5) located below the water level to a heated evaporator chamber (7), which opens above the water evaporating at the boiling point into the breathing gas line to a patient. A gas pressure equalizing line (12) is located between the heated evaporator chamber (7) and the intermediate storage unit (4).