  The internal magnetic field distribution in a mixed state of a cuprate
superconductor, Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ ($T_{\rm c}\simeq28.5$ K, near
the optimal doping), was measured by muon spin rotation ($\mu$SR) technique up
to 60 kOe. The $\mu$SR linewidth $\Lambda(B)$ which exhibits excess broadening
at higher fields ($B>5$ kOe) due to field-induced magnetism (FIM), is described
by a relation, $\Lambda(B)\propto\sqrt{B}$. This suggests that the orbital
current and associated quasiparticle excitation plays predominant roles in
stabilizing the quasistatic correlation. Moreover, a slowing down of the vortex
fluctuation sets in well above $T_{\rm c}$, as inferred from the trace of FIM
observed up to $\sim80$ K, and develops continuously without a singularity at
$T_{\rm c}$ as the temperature decreases.
