Long-term optical observation of the Io plasma torus provides a great deal of information on the plasma environment in the Jovian magnetosphere. We made imaging observations of emissions (λλ 673.1, 671.6 nm) from the Io plasma torus using a transportable telescope system for the period from 1997 to 2000. From the data we obtained intensity variations and average radial profiles and found that the plasma torus underwent systematic long-term variations between 1997 and 2000, indicating that Io's volcanic activity has been reducing over the years. We investigated the periodicity of torus emission intensity and confirmed that torus plasma is modulated at about 10.2 hours, corresponding to the so-called System IV period, and that the torus has no long-term System III periodicity.