Considering the adoption to fullscale measurements of ship wake velocity distribution, the authors have been developing a new system which is considered less expensive and handier than other devices, e. g. pitot-tubes and LDV. In this system, time history of space coordinates of a tracer is measured, using three TV-cameras, an image processer and a personal computer. Then three-dimensional velocity of the tracer is obtained by differentiating its space coordinates numerically. The measurement procedure is distinguished into two processes. At first the positions and directions of the respective cameras in the fixed three dimensional coordinate system are obtained by using some reference points whose space coordinates are known. Then, the time history of space coordinates of a tracer is obtained by using the above-mentioned camera positions and directions. Preliminary measurements were performed in air. Firstly, the accuracy of the space coordinate measurement was investigated. Then the accuracy of the velocity measurement was investigated, measuring the velocity of an object moving rotationally. The following results were obtained. (1) The accuracy of the space coordinate measurement, defined as the ratio of distance between the measured and actual space coordinates to that between the cameras and reference points, was about 0. 3% near the reference points and about 0. 6% in the whole measured volume. (2) The accuracy of the velocity measurement, defined as twice the standard deviation of the measured results, was about 4. 5%.