The dynamic structure factor of liquid Se was measured at 523 K using high-resolution inelastic X-ray scattering. Anomalous narrowing of the spectrum was observed at 15 nm-1, where the static structure factor S(Q) exhibits a weak shoulder, but the elastic part of the dynamic structure factor S(Q,E = 0) exhibited a strong maximum. The second frequency moment, which is estimated from only the quasi-elastic peak, is consistent with the motion of rigid six-atom clusters, while a formal agreement with the first-moment sum rule is preserved by the appearance of a weak intramolecular mode at 30 meV.