In order to understand the filtration mechanism in microfiltration, information concering cake concentration is important because the cake reduces the permeation rate. In this work, an ultrasonic measurement method of local cake concentration on the microfiltration membrane was developed by using a special array polymer transducer. This method was applied to microfiltration both with suspensions of yeast and polystyrene particles to study the effect of solidity of solute on cake compression. As a result, it was found that the yeast cake was compressible but that of polystyrene particles incompressible.