The electrooptic effect of the optical active crystals is studied in the experimental set of polarized light interference. In the polarized light interferometric experiment, the express equation of the relation between the emitted light intensity and the optical activity(I=A~2_0cos~2β-(π/λ)(n_l-n_r)l) is given. The express equation of the relation between the emitted light intensity and the electrooptic effect of the optical active crystals is also given, that is(I=A~2_0cos~2β-(π/λ)(n_l-n_r)l+(π/λ)(n_2-n_1)l.) According to these relations expressed by the equations, the electrooptic Q switch is made by using La_3Ga_5SiO_(14) crystal which is a typically optical active crystal. The Q switch works normally as those switches which have not optical activity. It may be used in medium output energy lasers to partly take the place of DKAP Q-switches.