The Japanese Ministry of Health, Labor, and Welfare has established guideline values for indoor air concentrations of eight volatile organic compounds (VOCs) and five semi-volatile organic compounds (SVOCs). In 2019, the Ministry updated the guidelines for three chemicals, including two phthalates, to reduce allowable concentrations. The Manual for the Measuring of Chemicals in Indoor Air proposes methods for analyzing these compounds; however, those for SVOCs, such as phthalates, are tentative. Therefore, in this study, inter-laboratory validation was performed in six facilities to propose a highly sensitive standard test method for the new guideline values of phthalate. The additional amount in this assessment was 5 ng, estimated based on the lowest concentration of the latest guideline values for phthalates (di-n-butyl phthalate: 17 µg/m³). The results showed that the average accuracy of each compound at each facility ranged from 73.7 to 150%, with bis(2-ethylhexyl) phthalate exceeding the validation criteria in the two facilities. However, the average accuracy across all facilities ranged from 96.5 to 119%, and repeatability and reproducibility varied from 1.4 to 17% and 10 to 17%, respectively. Therefore, the findings are largely consistent with the criteria for interlaboratory validation studies. This analytical method was proposed and published in the Manual for the Measuring of Chemicals in Indoor Air (Integrated Edition) as a standard test method for phthalates with guideline values established.