  Functional renormalisation group approach is applied to a imbalanced many-
fermion system with a short-range attractive force. Composite boson field is
introduced to describe the pairing between different flavour fermions. A set of
approximate flow equations for the effective couplings is derived and solved.
We identify the critical values of mass and particle number density mismatch
when the system undergoes a phase transition to a normal state and determine
the phase diagram both at unitary regime and nearby.
