A taste sensor using a lipid/polymer membrane, i.e., an electronic tongue with global selectivity, has been developed for objective evaluation of the taste of foods and beverages. Moreover, the taste sensor has been also contributing to safety of foods, e.g., the sensor membrane with strong hydrophobicity was used to detect sodium dodecyl sulfate (SDS), a negatively charged surfactant, which was generally added into the pesticide because of its strongly melting effect. An immersion process in monosodium glutamate (MSG) solution, called 'MSG preconditioning' was needed to obtain the change in membrane electric potential caused by adsorption (CPA) for sensor membrane before measurement. However, what happened to sensor membrane during MSG preconditioning is unclear. In this paper, we examined the relationship between the CPA value and the period of MSG preconditioning. The amount of adsorbed SDS and MSG was measured to figure out whether the CPA value is related to the amount of adsorption. As a result, with the precondition process progressed, the CPA values showed concentration dependence on SDS concentration, and increased to a peak by preconditioning for one day then decreased to a stable state after that. The amount of adsorbed SDS depended on the SDS concentration but did not change with the increasing of preconditioning time. In conclusion, we revealed that the most suitable time of MSG preconditioning for the membrane for SDS was one day. The CPA value was affected by both the surface charge density and the amount of absorption.