In this study, an analysis was made of the causes of variations in the results of Particle size distribution test (sedimentation analysis), liquid limit test, plastic limit test and unconfined compression test in which the coefficient of variation was nearly 5% or higher during proficiency testing of geomaterial tests. The analysis was made based on the results of a questionnaire survey conducted at the same time as the proficiency testing. As a result, it was verified that not only the experience of test engineers but also complying with standards, and checking, inspecting and correcting test apparatuses and measuring instruments are important to the improvement of the accuracy of geomaterial tests. It was also revealed that the maximum and minimum weights measured by a balance scale affect test accuracy.