THz electron spin resonance (ESR) measurement has been a powerful means to study magnetic systems due to the advantages of high spectral resolution, ESR measurement beyond the zero field splitting or the magnetic phase transition. We have been developing this THz ESR system, and we are now extending our developments to the multi-extreme THz ESR. Here the multi-extreme includes the high magnetic field, the high pressure, the low temperature and the micro-cantilever ESR. Using these advantages of THz ESR, we have studied perfect kagome lattice antiferromagnet Cr-jarosite, which has attracted much attention in the field of magnetism recently.