Groundwater flows which contribute significantly to slope failure are concentrated in highly permeable layers or continuous fracture zones and are referred to as groundwater vein streams. The underground temperature survey method is a simple technique for measuring the temperature at a depth of 1 m from the ground surface. The method not only enables to locate the groundwater vein streams, but also to determine the radius and the depth down to their center assuming they have a cylindrical section. A distance of about 6 m or less between the measurement points is essential for the accurate estimation of the radius and depth of groundwater vein streams. On the other hand, measurement conducted at 20 m intervals is sufficient for approximate location, and the difference between the temperature at a measurement point and the average temperature of neighboring measurement points provides a useful clue for the detection of a temperature change associated with ground water vein streams without considering influences of the topography and land cover. An underground temperature survey was conducted in a landslide area in Niigata prefecture for the study of the groundwater flow system. The depth of the groundwater vein stream estimated by this method agreed well with that determined by conductivity logging, permeability test and temperature logging conducted in a boring hole.