A method for detecting a foreign object by a wireless power transmitting system comprising a wireless power transmitting apparatus and a wireless power receiving apparatus, the method comprising: performing a first operation of foreign object detection; receiving, by the wireless power receiving apparatus, a power signal from the wireless power transmitting apparatus during a power transfer phase if no foreign object is detected by the first operation of foreign object detection; performing a second operation of foreign object detection during the power transfer phase; and performing a third operation of a fine foreign object detection by a temperature sensor of the wireless power receiving apparatus for detecting the foreign object which is not detected by the second operation of foreign object detection during the power transfer phase, wherein if the foreign object is detected during the power transfer phase, a power received at the wireless power receiving apparatus is limited by the wireless power transmitting apparatus, and wherein the first operation of foreign object detection comprises: receiving, by the wireless power receiving apparatus, a ping signal at an operating point from the wireless power transmitting apparatus which performs digital ping in a ping phase; generating, by the wireless power receiving apparatus, a signal strength packet indicating strength of the power signal at the operating point and transmitting the signal strength packet to the wireless power transmitting apparatus; generating, by the wireless power receiving apparatus, an identification packet indicating a unique ID of the wireless power receiving apparatus and configuration information of the wireless power receiving apparatus and transmitting the identification packet and the configuration information to the wireless power transmitting apparatus; measuring, an initial voltage at an output terminal in the wireless power receiving apparatus; and generating a foreign object state packet used to initiate or trigger the first operation of the foreign object detection in the wireless power transmitting apparatus, and transmitting the foreign object state packet to the wireless power transmitting apparatus, if the initial voltage is measured within the predetermined reference voltage range. performing a first operation of foreign object detection; receiving, by the wireless power receiving apparatus, a power signal from the wireless power transmitting apparatus during a power transfer phase if no foreign object is detected by the first operation of foreign object detection; performing a second operation of foreign object detection during the power transfer phase; and performing a third operation of a fine foreign object detection by a temperature sensor of the wireless power receiving apparatus for detecting the foreign object which is not detected by the second operation of foreign object detection during the power transfer phase, wherein if the foreign object is detected during the power transfer phase, a power received at the wireless power receiving apparatus is limited by the wireless power transmitting apparatus, and wherein the first operation of foreign object detection comprises: receiving, by the wireless power receiving apparatus, a ping signal at an operating point from the wireless power transmitting apparatus which performs digital ping in a ping phase; generating, by the wireless power receiving apparatus, a signal strength packet indicating strength of the power signal at the operating point and transmitting the signal strength packet to the wireless power transmitting apparatus; generating, by the wireless power receiving apparatus, an identification packet indicating a unique ID of the wireless power receiving apparatus and configuration information of the wireless power receiving apparatus and transmitting the identification packet and the configuration information to the wireless power transmitting apparatus; measuring, an initial voltage at an output terminal in the wireless power receiving apparatus; and generating a foreign object state packet used to initiate or trigger the first operation of the foreign object detection in the wireless power transmitting apparatus, and transmitting the foreign object state packet to the wireless power transmitting apparatus, if the initial voltage is measured within the predetermined reference voltage range. receiving, by the wireless power receiving apparatus, a ping signal at an operating point from the wireless power transmitting apparatus which performs digital ping in a ping phase; generating, by the wireless power receiving apparatus, a signal strength packet indicating strength of the power signal at the operating point and transmitting the signal strength packet to the wireless power transmitting apparatus; generating, by the wireless power receiving apparatus, an identification packet indicating a unique ID of the wireless power receiving apparatus and configuration information of the wireless power receiving apparatus and transmitting the identification packet and the configuration information to the wireless power transmitting apparatus; measuring, an initial voltage at an output terminal in the wireless power receiving apparatus; and generating a foreign object state packet used to initiate or trigger the first operation of the foreign object detection in the wireless power transmitting apparatus, and transmitting the foreign object state packet to the wireless power transmitting apparatus, if the initial voltage is measured within the predetermined reference voltage range.