We have observed clear indication of the transition from super- to normal-state conductivity in optimally doped YBa2Cu3O7−δ(Tc∼90K) in high magnetic fields up to 400T applied parallel to the CuO 2 plane (B||CuO2), using the electromagnetic flux compression system. In order to perform high-sensitivity magnetoresistance measurements in short-pulsed ultra-high magnetic fields, we developed a contactless radio frequency transmission technique. Therefore, a complete phase diagram of the upper critical field (H c2 ) for optimally-doped YBCO was constructed as a function of temperature on both cases of B⊥CuO 2 and B||CuO2. In the previous experiment, we found that the H c2 for B⊥CuO 2 would attain to 120T at T=0K, and follow the WHH phase boundary where quenching of the superconductivity was mainly governed by the orbital effect. On the other hand, it turned out that the observed H c2 for B||CuO 2 would reach around 250T at T=0K, which is consistent with the WHH theory that takes into account the spin Zeeman and spin–orbit effect.