<p>　Objective:　“Thickeners” as Foods for Special Dietary Uses are foods that have been approved by the government as thickeners suitable for people with dysphagia. In an approval examination, third-party organizations test whether the physical properties of a product meet standard criteria for approval. To ensure the fairness of the approval examination, the test results between laboratories should be highly reproducible. Thus, in this study, we investigated the reliability of the test method in an inter-laboratory study and discussed its fitness for purpose.</p><p>　Methods:　Three commercial xanthan gum-based thickeners were employed as samples. From December 2018 to February 2019, we conducted an inter-laboratory study involving ten laboratories. Each laboratory tested the physical characteristics of each sample according to the notification issued in March 2017. The results were evaluated to confirm the degree of agreement between the laboratories regarding analytical values and determinations.</p><p>　Results:　Regarding the performance requirements (solubility/dispersibility), serious discrepancies in the determinations between the laboratories were found. Regarding the performance requirements (temperature stability), although the determinations between laboratories were almost the same, analytical values were dispersed to some extent compared to the acceptance range, suggesting poor reproducibility. On the other hand, the test methods regarding viscosity requirements and performance requirements (temporal stability and resistance to saliva) seemed acceptable.</p><p>　Discussion:　In terms of the test methods issued in March 2017, the results for the performance requirements (solubility/dispersibility) were poorly reproducible, and thus the test conditions and determination criteria need to be realistically relaxed. Regarding the performance requirements (temperature stability), in order to improve the reproducibility, the addition concentration should be increased. Furthermore, to ensure the validity of the overall test methods, it would be helpful to add a stirring step immediately before the viscosity measurement and to set the addition concentration in the performance requirements to the design values indicated by the manufacturers.</p>