Mizoguti et al. (Chem. Phys. Lett. 321 (2000) 297) reported that amorphous carbon (a-C) contained in as-grown single-wall carbon nanotubes could be burned preferentially by using ultrafine gold particles and cationic surfactant, benzalkonium chloride (BKC). We confirmed this result and found additionally that the optimum concentration of the ultrafine gold particles and BKC were, respectively, 0.6 atom% and 7 g/l. We studied the roles of ultrafine gold particles and BKC in this phenomenon; the ultrafine gold particles catalyzed the oxidation of carbonaceous materials leading to the decrease of the burning temperatures. BKC had the function of homogenizing the a-C aggregation states, which resulted in the burning of a-C in a narrow temperature range.