A method of measuring the dynamic stress concentration factor αDmax in a fillet has not yet established. The reflection and transmission of stress wave during the propagation through a fillet result in complex stress states. We propose a new method for analyzing the dynamic stress concentration factor αDmax in a fillet utilizing the stress concentration factor αs and the reflection coefficient A obtained by a strain gauge on the incident strip plate. In this experiment, a square wave (a stress wave with a plateau of constant stress for long duration) is used. A strain gauge measures the reflection coefficient A. The factor αDmax in the fillet is analyzed using a dynamic photoelastic method. It is confirmed that αDmax is approximated by the following equation : αexDmax=(1+A)αs As a result, the dynamic stress concentration factor is obtained experimentally using the static stress concentration factor and the reflection coefficient.