This paper presents stance phase detection algorithm for pedestrian dead reckoning with a foot mounted IMU. Pedestrian Dead Reckoning (PDR) which uses gait pattern in pedestrian is one of representative systems. PDR is composed of step detection algorithm, step length estimation algorithm and heading estimation algorithm. A accurate step detection improves performance of Zero velocity UPdaTe (ZUPT) which used for step length estimation. This paper put emphasis on step detection algorithm and presents accurate step detection.