PURPOSE: To see images easily and enhance the monitoring properties by displaying a plurality of images indicating the kneading state of a mixer on one unit of high resolution CRT monitor.

CONSTITUTION: The kneading state of ready mixed concrete in a mixer 9 is photographed by a television camera 10 and an image signal is output. The variation amount (kneading load) of mixer power at the kneading time is sensed, and the slump value of ready mixed concrete is computed correspondingly to the variation amount and a graphic image signal is output. A composite signal output from the television camera 10 is converted to an RGB signal same as the image signal of slump value. The above-said signals are synthesized and displayed simultaneously on a monitor 3 for monitoring the kneading state of high resolution CRT 7.