Authors propose a method to identify the mass matrix of a large building structure by using a small active dynamic damper, or equivalently, an active tuned mass damper. We modify the acceleration feedback algorithm, which was once developed for improving the dynamic damper's performance, with a different objective. The advantage of the dynamic damper is its size: it is so small that there is a possibility that we could create an extra-small device for measuring the mass of a large structure. We review the physical meaning of the acceleration feedback, and then we use a single-degree-of-freedom model to explain how to operate the device to examine the weight of a primary structure. Then, we extend this method to a multi-degree-of-freedom model so that we can measure its effective modal mass with respect to the location where this device is placed. The identification of the mass matrix of a large structure can be completed as we shift the observing points and determine the associated effective mass. Several numerical studies are also carried out to certify the proposed method.