  We study how to measure the current structure of the process that $B$ meson
decays into two unstable fermions $\bar f_a$ and $f_b$ in model independent
way. We use the momentum distributions of subsequent decay products affected by
$\bar f_a f_b$ spin correlation. We have found the following: (1) We can
extract the absolute values of two effective coupling constants from the
opening angle between the particles decayed from $\bar f_a$ and $f_b$ (2) We
can extract the real part of the interference from the energy distribution of
one of the decayed particles from $\bar f_a$ or $f_b$. (3) No new information
can be obtained from the energy distribution of two decayed particles from
$\bar f_a$ and $f_b$. (4) The imaginary part of interference is extracted from
the azimuthal angle asymmetry of final-state decay products. (5) If only one of
two fermions is unstable, we can extract the real part of interference from
each of the energy distribution and opening angle distribution. We show several
simple examples.
