Information obtained from an inertial sensor attached on the foot would be useful in gait rehabilitation, healthcare and so on, because it provide various evaluation parameters such as stride length, inclination angle and gait event timings. The purpose of this study was to develop an estimation method of average walking speed with the inertial sensor attached on the foot. In this paper, average walking speed was estimated by estimated stride length and time of movement state of each stride during walking. The proposed estimation method of average waking speed was evaluated in comparing to measured walking speed with a stopwatch in 10 m walking test with healthy subjects. Mean difference of walking speeds between measured and estimated walking speeds was −0.60 ± 4.04% (−0.007 ± 0.048 m/s), and mean absolute value of the difference was 3.05 ± 2.71% (0.035 ± 0.033 m/s). Correlation coefficient between them was 0.986. Most of average walking speed were estimated with good accuracy, which were almost between ±10% and between ±0.1 m/s. These results suggested that the average walking speed estimation with an inertial sensor would be useful. In order to improve estimation accuracy for a specific subject and fast speed walking, in which stationary state between strides is very short time, is expected.