We have performed a systematic high-momentum-resolution photoemission study on ZrTe5 using 6-eV photon energy. We have measured the band structure near the Γ point, and quantified the gap between the conduction and valence band as 18≤Δ≤29 meV. We have also observed photon-energy-dependent behavior attributed to final-state effects and the three-dimensional (3D) nature of the material's band structure. Our interpretation indicates the gap is intrinsic and reconciles discrepancies on the existence of a topological surface state reported by different studies. The existence of a gap suggests that ZrTe5 is not a 3D strong topological insulator nor a 3D Dirac semimetal. Therefore, our experiment is consistent with ZrTe5 being a 3D weak topological insulator.