The vibrations which are produced in marine engines consist of, in general, very low frequency components, compared with those of audio frequencies. As, in the low frequency band, it is is very difficult to make a narrow band-pass filter by the use of usual inductances, capacitances and resistanses, we can not utilize a suitable frequency spectrum analyser, as used in audio frequency band, and must use tiresome numerical methods in order to analyze the spectra of such vibrations as mentioned above. As one of the methods to solve this difficulty, an active filter is introduced, which consists of a DC amplifier and its feed-back circuit with the R-C twin bridge. By using the active filters, a low frequency spectrum analyser is built, in which the center frequency of the filter is shifted step by step by 1/12 octave autmatically through selector switches and the frequency band is covered from 1 to 241. 8 Hz. The spectrum analyser is tested with the input signals which consist of sinusoidal waves whose frequencies and the ratio of their amplitudes are kown beforehand. The results are also compared with those obtained by Fourier expansion method. The following results are obtained. The low fequency spectrum analyser built in this experiment can detect almost exactly the frequency components contained in the test signals. The analyser can give the nearly exact ratio of amplitudes of the components contained. The numerical results give rather good values. However, the numerical ones give also higher harmonic components which are, in reality, not contained in the test signals. It can be expected that the low frequency spectrum analyserbuilt in this experiment may be used in practice.