Acid erosion of teeth was studied in two factories. The A was the textile factory possessing dyeing process using acids. The B was the metal product manufactoring factory possessing electroplating process. The control group was selected at the same factory not exposed to acids. The results were summarized as below. 1. The pH of saliva was the range of $6.6{sim}6.8$ in both factories. 2. The prevalence rate of erosion of teeth was higher in case group at the B. It was statistically significant. 3. The rate of erosion of incisor were the range of $12%{sim}16%$(case group), $7%{sim}21%$(control group) at the A and $10%{sim}20%$ (case group), $2%{sim}15%$ (control group) at the B. 4. The rate of erosion of degree 3 which is necessary for conforming, owing to occupational induction, were 8.9% (incisor: A) and 6.8% (incisor: B).