A quantum-mechanical 1/3 magnetization plateau and magnetic long-range order appear in the large-spin (5/2) substance SrMn3P4O14. Previous magnetization results suggest that the spin system consists of antiferromagnetic trimers that are weakly coupled with one another. We inferred that the magnetization plateau originated from discrete energy levels of the trimer. In order to confirm the discrete energy levels, we performed inelastic neutron-scattering experiments on SrMn3P4O14 powders. Observed magnetic excitations are consistent with excitations expected in the trimer model.