To confirm the reliability of trace-metal impurities analysis in industrial gases for semiconductors, a proficiency test was conducted for the analysis of sodium, chromium, manganese, iron and copper in a nitric acid solution at ppb levels. This testing was performed by comparisons between Japan and U.S. laboratories of the Taiyo Nippon Sanso Corporation (TNSC) Group jointly with the Chemicals Evaluation and Research Institute, Japan (CERI). The test samples provided by CERI were so stable that the relative standard deviations of the sodium, chromium, manganese, iron and copper concentrations were 7.3%, 0.3%, 2.0%, 0.4%, 2.7% for one and a half months, respectively. The test samples were delivered to three laboratories in Japan and the U.S. and analyzed by seven analysts and eight analytical instruments within one and half-month periods. Using a z score method, the reliability in the analysis of sodium, chromium, manganese and copper was generally confirmed. It was also confirmed that the analysis of iron was reliable enough, except for two data points provided by an analytical instrument and an analyst who did not normally perform metals analysis. Thus, these test results indicate that proficiency testing could evaluate the reliability of trace-metal impurities analysis precisely, and would be very useful in identifying trending errors in the analytical methods.