Abstract The purpose of this paper is to examine the mechanism of pore water pressure fluctuation and to grasp the environment of underground hydrology in the landslide by controlling drainage experiment. At first in this paper, the results of experiments on the pore water pressure fluctuation at the slip surface are explained. The second, the drainage effects of horizontal borehole are confirmed as quantitative grasp in drainage wells. Finally, the environment of underground hydrology in Tairasawa landslide area is cleared. The data was collected at Tairasawa landslide area during 1986-1987. The results are as follows:<BR>1) In this natural slope, the geological structure of hydrology around the No.1 drainage well has three confined groundwater strata with a three-stratum (upper, middle and lower) underground structure. It is proved that the confined groundwater in the upper and middle stratum is fluid mutually, but exclusively in the lower stratum. Furthermore, the dropping velocity of pore water pressure in the lower stratum is slow than other strata, and the pore water pressure fluctuation in the upper and middle stratum responds to rainfall.<BR>2) To compare with the upper stratum, the dropping volume of pore water pressure in the middle stratum (strong weathering rock stratum) at No.1 drainage well is larger, but the drainage volume is smaller. It is also found out that the increase of drainage volume and pore water pressure fluctuation due to rainfall are difference from weathering degree of rock stratum and depth of the observation borehole.<BR>3) The No.1 drainage well is a three-stage structure design. Each stage drains groundwater away from horizontal boreholes. The distribution of drainage volume has a tendency to concentrate groundwater at the lower stage. The drainage effects of No.2 drainage well is partial at some horizontal boreholes of lower stage, and the drainage volume of each horizontal borehole is clearly different.