A very slow earthquake or crustal deformation with a rise time longer than the period of the free oscillation of the Earth is difficult to observe on seismograms. However, we show that it can be detected from tide gauge records, because the free oscillation of an ocean basin such as the Japan Sea contains longer eigenperiods than those of the solid Earth. Actually, such an oscillation was excited by the Niigata earthquake of 1964 and recorded on tide gauges. Comparison of the tide gauge spectra of the free oscillation with the noise spectra shows that the free oscillation of the Japan Sea excited by a magnitude 7 class earthquake can be detected in the present tide gauge observation system. Further, a simulation for a very slow earthquake with a source process time of 1 hour shows that the slow event, which cannot be detected on seismograms, is detectable on tide gauges. However, the smaller the slip, the more difficult it is to detect the movement because of the limited dynamic range of the existing tide gauge system. If a slow deformation precedes a large earthquake, tide gauge records would be useful for earthquake prediction, so improvements in the observation system are desirable. ¿ American Geophysical Union 1988