Abstract This report is the results of at-power reactor noise experiments conducted at JRR-4. The magnitude and the frequency spectrum of the fluctuation of neutron flux are analysed under various operating condition. Three characteristic results are obtained: (1) the magnitudes and the frequency components in the relative fluctuation of the neutron flux do not alter at various power levels; (2) decrease of flow rate makes the neutron flux fluctuation weak; (3) the change of the shim plate position also varies the frequency components in the fluctuation. Based on these results the cause of the neutronic noise of this reactor is inferred to be the flow induced reactivity perturbation especially due to the lateral vibration of a shim plate. Next, the vibration itself is measured simultaneously with the fluctuation of neutron flux under reactor operating condition. The correlation between these signals are examined. Very high coherence and extremely similar frequency spectra of both fluctuation are obtained and it is definitely shown that the cause of the noise in JRR-4 is the lateral vibration of the shim plate under forced circulation of coolant. It is noticeable that the frequency spectrum of the shim plate vibration have a broad spectrum at low frequency region in contrast to the common understanding that the frequency spectra of vibration of core components have only discrete frequency components. The method of inferring source of noise from the behavior of the neutronic fluctuation under several power levels, coolant flow rates and control rod positions is proved to be useful if it is difficult to measure the source of noise itself.