We have investigated cavity-polariton condensation effects on the dispersion relation of the lower polariton branch (LPB) at 77 K in a CuBr microcavity by angle-resolved photoluminescence (PL) spectroscopy. The intrinsic cavity-polariton dispersion relations were characterized by the analysis of the incident light angle (in-plane wave vector k∥) dependence of cavity-polariton energies obtained using angle-resolved reflectance spectroscopy. It was found that a blueshifted PL band appears with a threshold-like nature as a function of excitation power density. The in-plane wave vector dependence of the blueshifted PL energy exhibits a flat dispersion relation in the k∥ range from 0 to ∼4.4 µm−1, which is clearly separated from the intrinsic LPB because of the large blueshift of ∼15 meV. We show that the blueshifted flat dispersion relation is possibly explained by the theory for a diffusive Goldstone mode peculiar to nonequilibrium cavity-polariton condensation.