Ground-based observations of [SII] 673.1 nm emissions from the Io plasma torus (IPT) showed a year-to-year decrease between 1997 and 2000. This phenomenon suggests that the electron and ion densities in the IPT decreased during the period, since an excitation source of the [SII] 673.1 nm emissions is an electron impact. Using Galileo plasma wave subsystem (PWS) data, local electron density in the magnetosphere (30-60 RJ) was investigated. The electron density in the middle and outer magnetosphere also showed a long-term decrease between 1997 and 2000. This concurrent long-term variation in the magnetospheric electron density with the IPT emissions verifies that the IPT gives influence to the magnetospheric plasma environment, as has been suggested since the Voyager era. Relative amplitudes of variations in the [SII] intensity and electron density were, however, different from each other.