The γ-mode eigenvalue problem is investigated to utilize an exponential experiment to validate nuclear data for reactor core analyses. The perturbation of the spatial decay constant γ by the bias of nuclear data is analyzed with the adjoint flux of the γ-mode eigenvalue problem. The adjoint flux at a phase-space position is found to be proportional to the amplitude of the neutron flux on a plane vertically distant from a source placed at the position. The implication of the adjoint flux is numerically demonstrated based on the diffusion theory. The perturbation theory relating the bias of the fission neutron emission to the perturbation of γ is preliminarily justified in the manner of the continuous energy Monte Carlo.