  A formula to calculate a norm overlap between Hartree-Fock-Bogoliubov (HFB)
states with the odd number parity (one quasi-particle excited states) is
derived with help of the Grassmann numbers and the Fermion coherent states. The
final form of the formula is expressed in terms of a product of the Pfaffian
for a neighboring even-even system (the zero quasi-particle state), and an
extra factor consisting of the Bogoliubov transformation matrix and the
anti-symmetric matrix in Thouless' HFB ansatz for the even-even system.
