Demand for using robots in unstructured environments has been increasing. When operating robots in unstructured environments, the robots often come into contact with objects intentionally or unintentionally. Since it is necessary to make contact with an object in order to execute complicated tasks, not only contact avoidance but also contact detection and stabilization of contact are required. This paper focuses on detecting contact during operation. A real-time force sensorless contact detection method based on applying dither and time-frequency analysis is proposed. Since the influence of dither on acceleration is attenuated when contact occurs, the proposed method judges the state of contact by monitoring the decrease in the dither frequency component of acceleration. The validity of the proposed method was shown by the improvement in contact detection performance verified through experiments.