Geomagnetic observations have intensively been carried out since 1976 in the Izu Peninsula, Japan, in an attempt to search for precursory and coseismic changes in the geomagnetic field. In order to enhance the significance of such changes, it is indispensable to clarify noises in the observed geomagnetic field. In this paper, noise problems in the Izu region are investigated, and it is particularly shown that rather systematic changes arise from the ocean tide effect. Since most of the observation sites in the Izu Peninsula are located close to the sea surrounding the peninsula, the total intensities observed at these sites must be affected by a magnetic field induced by movements of conducting sea-water under the presence of the geomagnetic field. A technique for reducing noises due to the oceanic tide is also investigated in this paper.