The responses of tide gauges to tsunamis are examined by in situ measurements at 40 stations in northeastern Japan. Recovery of water level in the tide well is measured after the water is drained or added to create a water level difference between the inside and outside of the wells. The recovery times for a 1 m water level difference, estimated from the observations, vary from station to station and range from 65 to 1300 sec. Tsunami waveforms on tide gauge records from the 1983 Japan Sea earthquake are corrected for the observed response. For those stations with the observed recovery times longer than 300 sec, the corrected waveforms differ significantly from the originals and reproduce the inundation heights near the tide gauge stations, indicating that the tide gauge system significantly distorted the tsunami waveforms. At such stations, the correction for the response is necessary for quantification of tsunamis. The recovery time is also computed hydraulically on the basis of the structure of the tide gauge system. The ratio of the observed time to the computed one ranges between 1 and 10 which is attributed to environmentally-induced change of the tide gauge system.