Faraday rotation and magneto-optical absorption spectral measurements were conducted on a spinel oxide, ZnCr2O4, a prototype of three-dimensional geometrically frustrated magnet. The measurements were carried out at temperatures down to 4.6K under ultra-high magnetic fields of up to 600 T. The ultra-high magnetic fields were generated by the electro-magnetic flux compression method. We obtained a precise magnetization curve up to a fully polarized phase, where the phase transition takes place above 400 T. The experimental magnetization curves were compared with those obtained by Monte Carlo calculations with an effective spin model including spin-lattice coupling up to fully saturated magnetization. The absorption spectral peaks of the intra-d-band transitions in Cr3+ ions as well as the exciton-magnon-phonon transition were used for monitoring the crystal and magnetic structures subjected to a strong external magnetic field. A novel magnetic phase was found prior to the fully polarized phase, which was clarified by the change in magneto-absorption intensity around 350 T. An umbrella-like magnetic structure was proposed to be the most plausible candidate for the novel phase. A physical analogy between the magnetic structures of ZnCr2O4 and the quantum phases of He-4 was discussed based on the similarity of symmetry breaking.