  We present the finite amplitude method (FAM) for superfluid systems. A
Hartree-Fock-Bogoliubov code may be transformed into a code of the
quasi-particle-random-phase approximation (QRPA) with simple modifications.
This technique has advantages over the conventional QRPA calculations, such as
coding feasibility and computational cost. We perform the fully self-consistent
linear-response calculation for a spherical neutron-rich nucleus 174Sn,
modifying the HFBRAD code, to demonstrate the accuracy, feasibility, and
usefulness of the FAM.
