Device for balancing fluids for an extracorporeal blood treatment device with a blood treatment device, wherein the device for balancing comprises: a balancing unit, a first inflow line leading to the balancing unit for delivering fluid to the balancing unit and a first discharge line leading from the balancing unit for discharging fluid from the balancing unit, a second discharge line leading from the balancing unit for delivering fluid from the balancing unit to the blood treatment device und a second inflow line leading to the balancing unit for delivering fluid from the blood treatment device to the balancing unit, the device for balancing comprising a monitoring device for detecting an improper balance of the fluid that is delivered to the blood treatment device and the fluid that is discharged from the blood treatment device, the monitoring device comprising an arithmetic unit that is configured such that the volume or quantity correlating to the volume of fluid that is delivered to the balancing unit is put in a ratio to the volume or quantity correlating to the volume of fluid that is discharged from the balancing unit, and that an improper balance is inferred on the basis of the ratio of the volume or quantity correlating to the volume of fluid delivered to and discharged from the balancing unit, and the monitoring device comprising a weighing device for determining the quantity correlating to the volume of fluid that is delivered to the balancing unit and of the fluid that is discharged from the balancing unit, and the device for balancing comprising a first container connected to the first inflow line for receiving fluid that is delivered to the balancing unit and a second container connected to the second discharge line for receiving the fluid that is discharged from the balancing unit, characterized in that the weighing device has a first scale for weighing the first container and a second scale for weighing the second container, and the arithmetic unit of the monitoring device is configured such that an improper balance is inferred if the weight of the first container differs from the weight of the second container in a predetermined time interval by an amount that is greater than a predetermined threshold. a balancing unit, a first inflow line leading to the balancing unit for delivering fluid to the balancing unit and a first discharge line leading from the balancing unit for discharging fluid from the balancing unit, a second discharge line leading from the balancing unit for delivering fluid from the balancing unit to the blood treatment device und a second inflow line leading to the balancing unit for delivering fluid from the blood treatment device to the balancing unit, the device for balancing comprising a monitoring device for detecting an improper balance of the fluid that is delivered to the blood treatment device and the fluid that is discharged from the blood treatment device, the monitoring device comprising an arithmetic unit that is configured such that the volume or quantity correlating to the volume of fluid that is delivered to the balancing unit is put in a ratio to the volume or quantity correlating to the volume of fluid that is discharged from the balancing unit, and that an improper balance is inferred on the basis of the ratio of the volume or quantity correlating to the volume of fluid delivered to and discharged from the balancing unit, and the monitoring device comprising a weighing device for determining the quantity correlating to the volume of fluid that is delivered to the balancing unit and of the fluid that is discharged from the balancing unit, and the device for balancing comprising a first container connected to the first inflow line for receiving fluid that is delivered to the balancing unit and a second container connected to the second discharge line for receiving the fluid that is discharged from the balancing unit, characterized in that the weighing device has a first scale for weighing the first container and a second scale for weighing the second container, and the arithmetic unit of the monitoring device is configured such that an improper balance is inferred if the weight of the first container differs from the weight of the second container in a predetermined time interval by an amount that is greater than a predetermined threshold.