PURPOSE: To enable the measurement of the volume of ready mixed concrete in a drum of a concrete mixer without changing the fundamental structure of the mixer, by providing a movable light-emitting means so that it faces the inside of the drum and by connecting the means to a control unit.

CONSTITUTION: At the opening 2 of a drum 1 of a concrete mixer, a movable beam-emitting unit 3 is provided, as a means of emitting a light such as laser beams, so that it faces the inside of the drum 1. The beam-emitting unit 3 is connected to a control unit 4, which comprises a pair of potentiometers 5 and 6, a processor 7 and a terminal 8. When it is wanted to know a residual amount and slump of ready mixed concrete in the drum 1, beams from the beam-emitting unit 3 are emitted, at the front end of the inner wall of the drum 1, toward two positions on the right and left sides R and L of at the upper-end portion of the ready mixed concrete, and the amounts of displacement in the (x) and (y) directions of the beam-emitting unit 3 at the respective positions are measured. The amount of content of the drum 1 of the concrete mixer is thereby detected.