Many studies have been conducted to investigate the change in human response under various experimental conditions. Usually, these experiments were conducted using many subjects and the inter-subject variation was evaluated. However, the intra-subject variation in human response is also necessary for understanding the change in an individual's physical response to whole-body vibration (WBV). The aim of this study is to investigate the intra-subject variation in biodynamic responses (both apparent mass and seat-to-head transmissibility) of a seated human exposed to vertical whole-body vibration over time. In the experiments, nine male subjects were exposed to vertical random vibration (0.2-0.3 m/s² in r.m.s.) in the 0-30Hz frequency range. The measurement variation was also evaluated, wherein the measurements were repeated five times without any change to form the “baseline” for each subject, and the intra-subject variations were evaluated by comparing their responses with these “baseline.” The intra-subject variation was examined from two different viewpoints: variation “within a day” and that “over several days.” To determine the intra-subject variation “within a day”, the five measurements were obtained at two-hour intervals on the same day. In the intra-subject variation “over several days”, the five measurements were obtained again, but at the same time of the day on five consecutive days. The results show that the intra-subject variations (both “within a day” and “over several days”) in biodynamic responses are larger than the “baseline.” However, when the variation “within a day” in biodynamic responses is compared to that “over several days,” no common trend is observed among subjects. Although the magnitude of intra-subject variation in biodynamic responses depends on each subject, both variations “within a day” and that “over several days” have a similar range of variation.