An acoustic emission is detected by a pickup, and its output is converted by a A/D converter into a digital signal, which is provided to a fast Fourier transform processor. The fast Fourier transform processor comprises a cascade connection of a plurality of operation stages, each composed of first and second input memories, a third memory having stored therein a rotation vector and an arithmetic unit for performing a Butterfly operation through using the pipelined architecture. In each stage one of the first and second input memories and the third memory are read out and the Butterfly operation is carried out on the pipelined basis. The operation result is written into that one of the first and second input memories of the next operation stage which has not been read out; thus all the operation stages perform a pipelined operation as a whole. The power of a spectrum obtained as the operation result of the last operation stage is calculated, obtaining a signal corresponding to the acoustic emission.