To investigate the effects of thickening agents, which may reduce tile risk of aspiration, on the swallowing threshold during food mastication, the physical properties of bolus immediately prior to swallowing during mastication of test food containing a thickening agent (xanthan gum) were measured in healthy ten adult participants (mean age 31 yrs). Two kinds of test food (test food C, cooked Koya-dofu as a control and test food X, cooked Koya-dofu containing 1% xanthan gum) were prepared. Each test food has a cubic shape (15×15×15 mm). Physical properties of test food and the test food bolus collected from the oral cavity immediately prior to swallowing were measured by method of the texture profile analysis. The number of chewing strokes until swallowing during mastication of the X was significantly smaller than that during mastication .of the C. Hardness, cohesiveness and gumminess of both the C and X boluses were significantly decreased during mastication. Adhesiveness of the X bolus was also significantly decreased during mastication, but the adhesiveness of the C bolus was significantly increased. These results suggest that the decrease in adhesiveness of bolus may be a main factor in initiating the swallowing during mastication of solid food containing the thickening agent.