We propose and implement a solution for applying an infrared radiation (IR) image sensor to step-on interface (SOI). SOI is a kind of natural human-robot interface what consists of a projector and a laser range scanner (LRG) sensor. And it enables any interactive touch applications on desktop or floor. We attempt to introduce an IR image sensor such as ASUS Xtion to SOI instead of LRG sensor. In this paper, we will describe the procedure that how to use the Xtion and detect touched point. We distinguish user's hand from background (surface) depends on depth data from ASUS Xtion, and detecting touching action when finger almost close to the background. The proposed processes involve IR depth image acquisition, seeking for hand and its contours by thresholding, recognition of touched areas and computing of theirs center position. The research enables ASUS Xtion be applied on SOI in a simple way. Moreover, this system can realize touch interaction on any surface.