To provide a highly reliable system for monitoring the volume ratio of each component by two-component mixing system online in real time.

This system suspends a ferromagnetic labeling material into a component B and detects the concentration by a sensor 31. When two components A, B are mixed by a mixing valve 29 and the mixed components are carried in a nozzle 35, the concentration of the labeling material therein is detected by a sensor 33. The read values of the two sensors are compared to calculate the volume ratio of the two components present in a mixing device. When the calculated ratio exceeds a prescribed range, an alarm is given, and a real time feedback is imparted to a supply control device to regulate the flow rate of each component.