A new measurement method to measure vibrational motions of objects is presented. The original principle is similar to the previous work that utilized a 3-facet mirror to obtain three dimensional positions and orientations of rigid bodies. While the previous work was presented for only stationary objects, in this paper, we newly investigate the feasibility of this method for dynamic applications. The 3-facet mirror that looks like a triangular pyramid having an equilateral cross-sectional shape. The mirror has three lateral reflective surfaces inclined 45 degrees to its bottom surface, and is mounted on the object whose motion is to be measured. As optical components, a He-Ne laser source and three position-sensitive detectors (PSD) are used. The laser beam is emitted from the He-Ne laser source located at the upright position and vertically incident to the top of the 3-facet mirror. The laser beam is reflected from the 3-facet mirror and splits into three sub-beams, each of which is reflected from the three facets and finally arrives at three PSDs, respectively. Since each PSD is a 2-dimensional sensor, we can acquire the information on the three dimensional position and orientation of the 3-facet mirror. From this principle, we can get the motion of any object simply by mounting the 3-facet mirror on the object. In this paper, the measurement principle and a series of experiments are presented. The experiments include measurements of vibrational motions of a piezoelectric actuator that moves the 3-facet mirror in a single axis. The experimental results are compared with those of a laser doppler vibrometer. Through the experiments, the proposed sensor is proven to be an effective means for measuring dynamic motions of objects.