In the present report, the authors describe the method and the results of the measurement with an improved radioisotope velocity meter, and gamma-gamma logging with radioisotopes, as an example, applied to ground water investigation in Fujinomiya near Mt. Fuji. With the new radioisotope velocity meter velocity and direction of ground water flow can be determined at the same time. In order to determine the depth to which the isotope velocity meter should be placed and to compare the geological column of two test borings, radioactive logging (60Co 1 mc) was made. Based on these results, the authors concluded that the lava strata with medium scattering intensity are of medium porosity and have the average ground water velocity, so it was decided to put the velocity meter at 42 meters depth. The computed ground water velocity was 1.316×10-3m/sec for No. 1 boring and 1.310×10-3m/sec for No. 2 boring. Radioisotope 131I solution (0.018 μc/ml) was used. Direction of ground water flow was determined by the dilution ratio. On the basis of the data obtained by the present measurement, the amount of the ground water which flows 1 m wide section was estimated.