Phospholipids such as phosphatidic acid exert a bitterness-masking effect without changing other taste qualities. In a previous study, we detected and qualified this suppression of bitterness using a taste sensor whose transducer was composed of several kinds of lipid/polymer membranes with different characteristics. The response to quinine hydrochloride decreased with increasing phospholipid concentration in a manner similar to human gustatory sensation. In this study, we observed the structural changes of a membrane surface using an atomic force microscope (AFM) and detected the adsorption of quinine hydrochloride and the masking effect of phospholipids. As a result of AFM measurements, we confirmed that the the masking effect was caused by blocking the adsorption of bitter substances onto to the membrane.