To realize the production of ready-mixed concrete without deteriorating its quality by a method wherein the kneading of the ready-mixed concrete under proper design is performed by quantitatively and uniformly calculating the concentration of sludge water by computer image processing at the pouring of the sludge water and of the mixed water of the water and the sludge water.

A CCD camera 4 is installed at a water metering bin 3 so as to measure the color of the sludge water at the pouring in the water metering bin 3 and the size of water drop and represent them in numerals. At the same time, the color and transparency of the mixed water after being poured in the metering bin are taken in with a CCD camera 5 as image data and then represent them in numerals. These numerical data are compared with the values corresponding to the sludge concentration stored in a computer memory 7 in advance so as to calculate the sludge concentration of the mixed water in order to calculate a design for correcting the amounts of cement 14, sand 12 and gravel 13.