This article suggests a technique to make back of the hand (BOH) a track pad by attaching a distance ranging sensor on the side of a smart watch. It does not only estimate the position of a finger on BoH but also distinguishes the presence or absence of distance between the operating finger and BoH, which functions as an operating surface. Distance data and signal strength data are obtained by distance ranging sensor array, and the presence of distance is distinguished by machine learning. Our performance evaluation shows the accuracy of finger position estimation is 4.2 mm (SD = 1.2 mm).