This paper introduces a new instrument for measuring groundwater velocity in a borehole using a laser Doppler velocimeter (LDV) technique. The major improvement of the proposed LDV system is that the probe can be inserted into a borehole that has a minimum diameter of 10 cm and that a disk-type rotating grating is used as a frequency shifter for low velocities in order to determine the velocity sign. The speed of the glass plate passing through the LDV measurement point is calibrated to demonstrate the accuracy of the new LDV system. From a comparison with the true constant speed of the glass plate, we confirm that the new LDV system can measure low speeds in the range from 10-5 to 10-2 cm/s at a measurement error of less than 8%. Laboratory experiments were carried out to investigate the performance of the new LDV system for the horizontal velocity measurement of groundwater in a borehole. The result obtained by the proposed LDV system shows that the horizontal velocity at the center of an unscreened borehole is 3 times larger than the seepage velocity in a confined aquifer. This result agrees with the theoretical solution for groundwater flow in a borehole derived by Sano (1983). The validity of the theoretical solution is experimentally verified in the laboratory using the current LDV system. Although the detectable minimum velocity in a borehole is limited by the velocity fluctuations due to the Brownian motion of scattering particles, results obtained in the experiments using 0.804-&mgr;m polystyrene particles show that the LDV system described in this paper can measure with a sufficient accuracy both the magnitude and the direction of groundwater velocity down to 10-4 cm/s in a borehole. ¿ American Geophysical Union 1993