We made marine surveys of the geomagnetic total intensity in the northeastern Izu region, aiming at observing a possible change in the total intensity distribution associated with a magma body which may have been involved in the submarine eruption of July 13, 1989. We first compared the result of our survey made on October 16 and 17, 1990 with the result of the survey made on October 16, 1989 by the Maritime Safety Agency. Then it turned out that no significant change appeared over the submarine eruption site during the one-year period after, the eruption. The comparison between the results of surveys made on October 16 and 17, 1990 and on May 21, 1991 also shows no appreciable change over the eruption site. We may conclude, therefore, that a large magma body is unlikely to be involved in the eruption itself, as also suggested in crustal deformation and gravity data interpretations.