A communication method of a wireless power transmitter for transferring power in a wireless manner, includes allocating a slot among a plurality of slots to a first wireless power receiver, the slot being allocated to the first wireless power receiver for acquiring information of the first wireless power receiver while wireless power is transferred to the first wireless power receiver; transferring the wireless power to the first wireless power receiver by the wireless power transmitter; detecting a second wireless power receiver by the wireless power transmitter during the wireless power transfer to the first wireless power receiver; and generating a collision related signal based on a frequency shift keying (FSK) such that a collision resolution mechanism is executed by each of the first and second wireless power receivers respectively when first information generated by the first wireless power and second information generated by the second wireless power are in a collision within a first unallocated slot among the plurality of slots, the first information and the second information being generated based on a load modulation.