PURPOSE: To measure a slump value easily and efficiently, by installing a blade to a rotary shaft which has passed through the center of a center shaft of a drum, and has been extended out into the drum, and detecting the resistance which this blade receives from ready mixed concrete.

CONSTITUTION: In the center of a center shaft 2 of a drum 1, a rotary shaft 10 is provided so as to pass through. Also, to the tip of the inside of the drum 1 of this rotary shaft 10, a blade 11 consisting of 2 pieces or 4 pieces, etc. is installed, and to the opposite side, an output shaft of a vane type oil pressure motor 13 is connected through a coupling 12. In this state, when the oil pressure motor 13 is rotated at a constant speed and ready mixed concrete is agitated, the blade 11 receives resistance in accordance with softness of ready mixed concrete, and by detecting its resistance, a slump value can be measured easily and efficiently.