The aim of this study is to develop a three-dimensional touch interface for mobile devices, specifically a touch interface for detecting fingertip force. This interface consists of a conventional touch interface and an electromyogram (EMG) amplifier. The fingertip force during manipulation of the touch interface is estimated from the EMG measurement. We develop a method for obtaining fingertip force information using an EMG, while the two-dimensional position of the finger is measured using the conventional touch interface found in mobile devices. Further, we evaluate the validity of our newly developed interface by comparing the fingertip force estimated using our proposed method with the fingertip force measured using a force sensor. Lastly, we develop an application using our interface.