We have considered a boson system interacting with its heat reservoir, where the boson–reservoir interaction includes
not only a non-adiabatic part but also an adiabatic part, and have generalized the non-equilibrium thermo-field dynamics
proposed by Arimitsu and Umezawa [8–10] to the case of a non-bilinear unperturbed Hamiltonian which includes not only a
bilinear part but also a non-bilinear part with momentum mixing.We have obtained the forms of the quasi-particle operators
for a semi-free boson field, and have evaluated the form of the two-point Green’s function for the semi-free boson field. We
have derived a form of the admittance for the boson system using the TCLE method formulated in terms of the generalized
non-equilibrium thermo-field dynamics, where the admittance includes effects of the initial correlation and memory for the