In this study, we realize a next-generation extended input interface that can identify the fingers used for operating mobile devices through bioelectromagnetic response sensing. First, the impedance characteristics of the five fingers were elucidated through an experiment involving different subjects. Furthermore, we discussed the possibilities of finger identification and subsequently focused on the differentiation between the thumb and the index finger. Next, we evaluated the differences between the bioimpedance characteristics of each finger for several subjects and clarified that the bioimpedance scale relation between fingers is the same for a particular subject. Moreover, the bioimpedance of a single subject was measured by letting the subject change the manner by which they hold a case or touch an electrode, and the results demonstrated that the bioimpedance characteristics of the thumb and index finger can be clearly distinguished even in discontinuous measurements. Finally, we implemented an identification algorithm in which the bioimpedance of the index finger from previous measurements was taken as the reference and performed evaluations to verify if a highly accurate differentiation of the thumb and index finger is possible. The results demonstrated that the proposed interface can be used in practical applications. Additionally, these findings highlight the potential of various applications utilizing bioelectromagnetic response sensing.