To obtain a detecting apparatus by which an abnormal sound can be detected precisely even in the transition of a rotating body.

Data on the sound pressure of a rotating body is obtained by a first detection part 11 which is composed of a microphone installed near the rotating body. The speed of rotation of the rotating body is detected by a second detection part 12, detection data is input to a pulse-interval extraction part 13, and pulse which are proportional to the speed of rotation is obtained at its output. The pulses are input to a synchronous detection part 14. When the data on the sound pressure which is detected by the first detection part 11 is sampled, the data on the sound pressure detected by the first detection part 11 is sampled so as to be synchronized with the interval of the pulses obtained by the second detection part 12, and sampling data is obtained. The sampling data is extracted by a sampling-data extraction part 15, and the sampling data is input to a judgment and processing part 18 via a data input part 16 and a data storage part 17. The data is processed by the judgment and processing part 18, the parallelism of tangent lines is evaluated, and whether a noised is added to the data or not is discriminated by a judgment result part 19 so as to be output to an output part 20.