A tropospheric delay is known to give non-negligible error to measurements in precise displacement monitoring using Global Positioning System (GPS), when difference in height between GPS antenna points is large, or more than a few tens of meters. One effective way to reduce such error, in the case of short baselines (less than a few kilo-meters), is to process GPS signal taking mathematical tropospheric model such as the Modified Hopfield model, and meteorological data near the measurement site. However, under the condition of large height difference between antenna points, some amount of error still remains after the correction, especially in the seasons that weather changes a lot. Therefore, this paper proposes a new method to correct tropospheric delay error more easily and effectively than the correction method using mathematical tropospheric model and meteorological data. In our method, tropospheric delay error is directly measured at a fixed point, and the measured error is proportionally allocated to the other measurement points depending on the height differences between the reference point and the measurement points. This proposed correction method was applied to a field measurement that had continued for more than five years at an open-pit limestone quarry, which had nine GPS antennas with a maximum height difference of 209 meters. The results showed that the proposed correction method successfully removed the tropospheric delay errors better than the conventional method. Furthermore, according to a measurement performance test that was carried out in the same site, 5-millimeter displacement given to an antenna in height direction could be detected within 1-millimeter accuracy.