We evaluated the microbiological safety of bottled mineral water products commercially available in Japan. Of 10 bottled mineral water products manufactured in Japan, no bacteria or fungi were detected in 9 (90%), but 1 (10%) contained 1.8×10³ colony-forming units (cfu)/mL. Of 12 bottled mineral water products manufactured in the EU, 11 (91.7%) contained 23–3.5×10⁴ cfu/mL. On the other hand, of 5 bottled mineral water products manufactured in North America, 2 (40%) contained 2.3×10²–2.5×10³ cfu/mL. The detected microorganisms were glucose-nonfermentative Gram-negative bacilli such as Brevundimonas vesicularis, Moraxella spp., and Burkholderia cepacia, but Pseudomonas aeruginosa was not detected in any product. For immunocompromised host patients being managed in ultra-clean rooms, the examined bottled mineral water products manufactured in Japan, except 1, were microbiologically safe.