To evaluate the suppressive effect of a perforated plate on the sloshing of water in a tank during an earthquake, the behavior of water sloshing in a rectangular tank installed with a perforated plate was studied. Plates with various combinations of opening fraction and hole diameter were installed at the center of the tank, the frequency was changed at a given interval, and the wave height in the tank was measured. To accommodate cases where a perforated plate cannot be installed at the center of tank, off-center positions were also tested. For perforated plates with a relatively large opening fraction, the predominant wave height was close to the natural frequency of sloshing based on the tank width, and the suppression of wave height increased with decreasing opening fraction. For perforated plates with a relatively small opening fraction, the natural frequency of sloshing changed to one based on the half-width of the tank, and the suppression decreased with decreasing opening fraction. Under the present range of experimental conditions, the suppression of wave height was maximal at the opening fraction of 15%, and it increased slightly with decreasing hole diameter at this opening fraction. Installation of the perforated plate at a slightly off-center position had little effect on the observed suppression of wave height.