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MnBi₂Te₄ is one of the most promising candidates of intrinsic magnetic topological insulators (MTIs) that displays nontrivial band topology with an intrinsic antiferromagnetic state. However, there are inconsistent observations on the existence of the exchange gap of MnBi₂Te₄, partially due to the highly electron-doped nature of the MnBi₂Te₄ crystals and local inhomogeneity from the native defects. Here, we tailor the material through Sb substitution to reveal the gapped surface states in MnBi_2−xSb_xTe₄. By shifting the Fermi level into the bulk band gap, we access the surface states and show a band gap of 50 meV at the Dirac point from quasiparticle interference measured by scanning tunneling microscopy (STM). Surface-dominant conduction is confirmed through transport spectroscopy measured by multiprobe STM below the Néel temperature. The surface band gap is robust against the out-of-plane magnetic field despite the promotion of field-induced ferromagnetism detected by in situ magnetostriction measurement. The realization of bulk-insulating MTIs with the large exchange gap offers a promising platform for exploring emergent topological phenomena.