  The investigation of high-temperature superconductors under high magnetic
fields is one of the most important topics in condensed matter physics. For
YBa$_2$Cu$_3$O$_7$ (YBCO), the measurements of magnetoresistance under a high
magnetic field are technically challenging because the required magnetic field
($B$) is 100 T class. The low temperature (from 52 to 150 K) magnetoresistance
is measured in optimally-doped YBCO thin films under the condition
$B$$\parallel$CuO$_2$-plane up to 103 T by employing the single-turn coil
technique and a radio frequency reflection method. The electrical resistivity
$\rho$ exhibits $B$-linear behavior in the normal phase in the high magnetic
field region. The field slope coefficient $\beta$ (=$d\rho/dB$) becomes
converged at low temperatures. The convergency of $\beta$ below $T_c$ indicates
the field-induced strange metal phase, which is determined by the quantum
critical fluctuation at high magnetic fields. The $\beta$ value difference
under the different directions of the magnetic field suggests the strong
anisotropy in quantum critical fluctuations in the strange metal phase of YBCO.
