PURPOSE: To make the existence and position of a heterogeneous layer in suspension detectable in an easy manner by measuring the dropped variable of a weight lowered as one body in the suspension and the extent of gas purge pressure in a position in the suspension by means of a gas purge nozzle.

CONSTITUTION: A nozzle 3 is dipped in suspension 5 together with a weight 1 and then this nozzle 3 is going down in the suspension as blowing off bubbles of gas 8 from its noze. In this case, while solid portion content in the suspension is almost homogeneous, the extent of purge pressure is proportioned to depth from its level, but if it gets to a heterogeneous layer 6, the purge pressure suddenly goes up, and at a time when it arrives at a sedimentary layer 7, this purge pressure is conspicuously heightened, while the weight 1 is no longer lowered. Here, a distance of the weight 1 lowered is measured (2) by way of rotating a drum with, for example, a wire to lower the weight 1 and thereby integrating the number of pulses to be generated by a servomotor according to this rotation. With this constitution, any variation in the depth direction of the solid portion content in the suspension is easily measurable and, what is more, the existence and position of the heterogeneous layer is thus detectable.