Falling is a common problem in the growing elderly population and fall risk assessment tools are needed for community-based fall prevention program. To improve motor and cognitive function simultaneously, rhythmic stepping exercise (RSE) has been proposed and we have developed a measurement system using a laser range sensor (LRS). However, there is to be danger of falls during the RSE depending on the motor function of the participant. Therefore, it is necessary to determine whether the participant is able to perform the RSE or not based on the evaluation of motor function. A one-leg standing test can evaluate the motor function of the participant in a short period of time. The aim of this study is to propose a method of measurement of one-leg standing for the qualification test of the RSE using an LRS. In the RSE measurement system, the LRS installed at the shin height to obtain the trajectory of both legs during the RSE. However, it is difficult to measure a start (foot-off) and end (foot-contact) time of one-leg standing from the two-dimensional distance data obtained by the LRS directly. To solve the problem, we propose a method to detect the behavior of foot-off and foot-contact based on not only the position and speed but also the width of the raising leg which are obtainable in the two-dimensional data. From the experimental results with elderly people, it was confirmed that the proposed method could measure the foot-off and foot-contact time in one-leg standing.