To ensure that elderly members of society are able to maintain a quality of life that allows them to live independently, it is important that impending frailty, which occurs between the state of begin healthy and needing nursing care, be detectable at an early stage. The purpose of this study was to establish a system to continuously measure of elderly people as they go about their daily lives at home and to detect early signs of pre-frailty. This study focused on the decrease in walking speed of elderly people in pre-frailty, designing a means of measuring walking speed using a non-wearable passive infrared (PIR) sensor that performs equivalent accuracy with conventional manual measurement. The walking speed measured by this system with an average error of 1.7% at five meters from the walking speed measured by the gating method. Further testing confirmed that walking speed could be measured with an average error of 1.5%, even at one meter. The system is feasible, cost-effective, and can be easily installed in homes for continuous measurement of the walking speed of elderly people.