It is very difficult for dental professionals to objectively assess brushing skill of patients. Although oral health is one of the most important factors for general health, dental professionals cannot provide clear guidance on tooth brushing to patients. To perform the appropriate instruction of brushing to patients and the progress of brushing skill, dental professionals need a method of quantitative evaluation and a clear step-by-step index of brushing skill in patients.In this study, we focused on the reciprocatory motions of the toothbrush during brushing. Toothbrush motion performed by a dental hygienist for 15 seconds was captured using a motion-capture system that could continuously calculate the three-dimensional coordinates of an object's motion relative to floor. The dental hygienist performed the tooth brushing on the buccal side of the right and left upper molars. The frequency and power spectrum of the toothbrush motion were calculated and analyzed statistically.Study conclusions :1)The motion capture system accurately and continuously captured the three-dimensional coordinates of the motion of the toothbrush and was an effective method for acquiring the motion during tooth brushing.2)Although the movement for the right side of the upper molars was more unstable than that of left side, it was confirmed quantitatively that the dental hygienist had an individual distinctive rhythm during tooth brushing.We suggest that an individual's frequency during tooth brushing might provide a numerical index to assess the brushing skill of patients.