A protecting tube 51 is disposed inside the mixed phase container 1 with one end fixed to the inside of the mixed phase container 1 and the other end projecting freely into the mixed phase container 1. A detecting rod 60 is inserted into the free end of said protecting tube 51 and fixed to the protecting tube 51 with a part of the detecting rod 60. Magnets 61 are mounted in a uniform magnetic flux on the free end of the detecting rod 60 and is movable about a point F1 of the detecting rod 60 as a fulcrum. Agglomerates 20 in the mixed phase container 1 are detected by measuring magnetic flux fluctuations produced by movement of the magnets 61.