The present invention relates to the dust sensor and method in the wireless power transmission system. Such this specification discloses the wireless power transmitting device including the second core block equipped in the wireless power receiving device and the first core block which transmits the wireless power with the wireless power receiving device by combining with the magnetic induction or the self resonance, the control unit which is connected to the first core block and it is connected to the electric driving unit in which the first core block transmits the wireless power but applying the necessary AC signal, and the electric driving unit and produces the control signal controlling the AC signal, and the current measurement unit which measures the current flowing in the first core block and the control unit interprets the current but obtains the current measurements which changes to the suitable value. The foreign substance intruding into without the help of the wireless power receiving device between the wireless power transmitting device and wireless power receiving device can be recognized as for oneself and in case the foreign substance is sensed the wireless power transmission is discontinued or it tells the user to remove the foreign substance.