A breathing gas system and water trap (1) is improved in respect to the reliability of operation and has an emptying device. The water trap (1) has a gas inlet (4), which meets a first water separating membrane (5) via a connection line. The connection line leads into a water tank (20) located deeper in the incoming flow direction from the first water separating membrane (5) and into a gas measuring device (2) with vacuum on the discharge side from the first water separating membrane (5) from the water trap (1). The water tank (20) has a rinsing gas flow line, which is arranged above the liquid level, leads upward via a gas-permeable membrane (6) and is likewise connected to the applied vacuum of the gas measuring device (2). The water tank (20) has a water transport line for emptying the water tank (20), which water transport line extends into the liquid and is provided with a downstream vacuum via a nonreturn valve (7) and a downstream liquid pump (17) or via a solenoid valve (21). The water trap (1) is equipped with a filling level detection system (8, 9, 22; 8, 9, 13; 8, 10), which is connected to a control and analyzing unit (3). The control and analyzing unit (3) is connected to the liquid pump (17) or to the solenoid valve (21), so that the water tank (20) can be emptied as a function of the signal of the filling level detection system (8, 9, 22; 8, 9, 13; 8, 10).