This paper deals with the development of a new method for detecting the number of cloth. It is necessary to detect one or plural number of sheets to avoid mishandling. This detecting sensor utilizes the variation of the phase of transfer function within entire acoustic system. The impedance of sheets varies with the number of sheets. Characteristic acoustic impedance and complex wave number define the material peculiar and the most fundamental characteristic. It is useful for the detection of the number of cloth to know characteristic impedance of cloth. So that, we tried to measure characteristic impedance and wave number. The measured impedance of sheets by transfer function method was introduced into theoretical analysis. The results of experiments and theoretical analysis show fairly good coincidence.