By setting a hypothetic weight value and a virtual operation mode and executing the virtual operation mode, the zero point of a reference value for calculation of the weight value of metered materials of metering hoppers is corrected by using the hypothetic weight value; even in an empty state that metered materials are not applied to the metering hoppers, it is presumed that metered materials with the hypothetic weight value are supplied, and a combined scale is operated in a simulated manner.