Regularity of Ga⁺ primary ion TOF-SIMS fragment pattern of inorganic compounds is discussed. For an inorganic compound as formulated M-A, where the valence of cation M is +n and that of anion A is −p, the molecular formulas of appeared TOF-SIMS fragments are MxAy, which satisfy the rule nx ≥ (py + 1) for positive ion fragments and nx ≤ (py + 1) for negative ones. For example, for oxide fragments of molecular formula, MxOy (the valence of M is +n), the fragment pattern obeys the rule nx ≥ (2y + 1) for positive ions and nx ≤ (2y + 1) for negative ones, respectively. The regularity of TOF-SIMS fragment patterns of sulfides, nitrates, sulfates, etc. is discussed.