In Ga⁺ primary ion TOF-SIMS, the fragment patterns from thin oxidized metal surfaces appear obeying the already proposed rule. The fragment ions M_xO_y (M: metal) appear in qx ≥ 2y+1 for positive ions and qx ≤ 2y+1 for negative ions (q: valence of M). This paper shows the possibility of the thickness estimation of thin oxide films (SiO₂) on Si, from the relative intensity of typical fragment ions from thin oxides or the substrate as identified by the proposed rule in Ga⁺ primary ion TOF-SIMS spectrum.