In the terahertz frequency range, it is difficult to achieve the control of both light-polarization and intensity by external forces. This difficulty prevents the practical application of future terahertz devices. Here the research team, led by Professor Yoshinori Tokura, demonstrated the gigantic change of light-polarization and intensity at terahertz frequencies. They use the spin-dependent optical activity and the magneto-chiral effect, both of which are expected to be induced by the chirality of the specific magnets. The observed rotation of light-polarization reached 90 deg. when the terahertz light propagates the crystal with a thickness of 1 mm. The strength of the magneto-chiral effect becomes gigantic approaching 100%, as characterized by the difference between parallel and antiparallel absorption of the crystal. The present findings can provide the guiding principle of new terahertz devices.