  We present a framework to analyze effects of new physics beyond the standard
model on $B$-$\bar B$ mixing and $CP$ violation in $B$ decays in a
model-independent manner. Assuming that tree level decay amplitudes are
dominated by the standard model ones, new physics contribution to the $B$-$\bar
B$ mixing can be extracted from several measurements at $B$ factories. Using
this framework, we show the present constraint on new physics contribution to
the $B$-$\bar B$ mixing, and illustrate constraints expected to be given by
future experiments at $B$ factories. We also point out a possibility that $CP$
asymmetries in $B\rightarrow\psi K_S$, $B\rightarrow\pi\pi$, and $B\rightarrow
DK$ modes look consistent with the standard model, even if a large new physics
contribution is present in the $B$-$\bar B$ mixing.
