Abstract: A flow rate of milk during milking is measured to obtain a detection value (a second detection value) D.sub.N having a correlation with respect to the whole amount of milk during a fixed sampling period Ts at any time and a first detection value D.sub.N-1 having a correlation with respect to the whole amount of milk during the fixed sampling period Ts at a time earlier by a fixed time To than the time for the fixed sampling period Ts for the second detection value. A variation ratio C.sub.N of the second detection value D.sub.N to the first detection ratio D.sub.N-1 is calculated by using an equation of (second detection value D.sub.N)/(first detection value D.sub.N-1) and the variation ratios C.sub.N (C.sub.1. . . ) are monitored to detect the variation degree and the variation direction (increasing and decreasing direction) of the flow rate of milk so that the milking condition is monitored when a cow is milked by a milking machine.