This report proposes a method for measuring free-falling nappe distance and depth of debris flow in order to estimate more accurate debris flow velocity. The velocity was calculated according to a semi-theoretical equation. Nappe distance and depth were obtained using a laser range finder installed into Arimura River in Sakurajima, Japan. In the debris flow that occurred on June 21, 2014, the nappe distance（L）and depth（he）of debris flow was obtained, and the nappe distance was approximated by L＝10.1 he－0.17, where observed maximum he is 0.7m. A comparison of the velocity estimated from the semi-theoretical equation and collected using the ultrasonic velocity sensor showed that the calculated velocity is valid. The proposed method is capable of measurement even during bad weather, and it is possible to estimate the debris flow velocity at a high time resolution.