Note on the field theory of neutrino mixing

The problem of the diagonalization of the flavor-neutrino propagator mat rix is investigated in the theory with flavor-mixing mass terms in Lagrangian. For this purpose we examine one-pole structures of flavor-neutrino propa gators, leading to physical neutrino masses, and discuss the relation of the pro pagator diagonalizaion to the diagonalizaion of the mass matrix in Lagrangian. In connection with the paper by Blasone et al., it is pointed out that t here is no compelling reason for fixing the mass parameters, althought this fixi ng is necessary in order to construct the flavor Hilbert space.