Extensive usage of chrysotile asbestos has resulted in the remains of large numbers of chrysotile asbestos-containing buildings to be surveyed. We have recently developed a simple dyeing method for detecting chrysotile asbestos in building materials, which involves pretreatment with calcium-chelating agent and dyeing treatment with magnesium-chelating organic dyes. In this study, we further developed a method which eliminates dyed asbestos substitutes containing magnesium, potentially present in building materials. In the new method, post-treatment with formic acid was conducted to dissolve the non-chrysotile asbestos materials in order to delineate dyed chrysotile asbestos. The calcium-masking process was also shown to be an essential process even when the post-treatment was conducted. It was shown that the new method developed in this study may enable us to dye chrysotile asbestos only without detecting asbestos substitutes in building materials.