The present study examined the relationship between the physical properties of thickening liquid foods and the condition of swallowing by elderly persons without awareness of difficulty in swallowing as subjects. In order to study how the subjects swallowed the samples, the authors simultaneously employed two methods: sensory evaluation based on a ranking method, and videofluorographic examination of swallowing. Thickening liquid food samples of three different stages of hardness were employed for the study: sample with hardness of 2×10² N/m² (simple contrast medium sample), as an example of a sample prepared without addition of commercial thickening agent; as well as a sample with hardness of 4×10² N/m² (similar in hardness to stirred commercial plain yogurt) and a sample with hardness of 1.6×10³ N/m² (similar in hardness to mayonnaise), as samples prepared with the addition of commercial thickening agents. The outcome of the studies revealed some cases of aspiration (including inflow into the larynx) among subjects when given the sample with hardness of 2×10² N/m². This sample demonstrated a low coefficient of viscosity and Newtonian flow, and was characterized by the shortness of timing of elicitability of swallowing reflex, which caused a relative delay in elicitability of swallowing reflex, thus resulting in aspiration in some subjects. Prolongation of transfer time in the oral cavity was observed for the sample with hardness of 1.6×10³ N/m², which was characterized by a high coefficient of viscosity, high adhesiveness, and more prominent in-mouth feeling of stickiness. These findings further suggest that, even among elderly persons without awareness of difficulty in swallowing, eating thickening liquid foods that have been adequately thickened is more likely to reduce the risk of aspiration, than eating smooth and free-flowing liquid foods.