[en] A suppression effect of coherent synchrotron radiation due to conducting boundaries was observed by using the Tohoku 300 MeV Linac. Electron bunches, whose energy was 50 MeV and longitudinal length was 1.65 mm, were bent by the magnetic field of 68.6 mT between a pair of finite parallel aluminum plates with a variable gap in a vacuum chamber. As the shielding gap became narrow, the intensity of coherent synchrotron radiation was enhanced at first and then drastically suppressed. The results can be qualitatively explained by the theory of suppression effect obtained by Nodvick and Saxon and applicable to the intense and short-bunched accelerators. (author)