A wireless power transmitter , comprising: a communication/control unit configured to perform a negotiation for a first available power indicator with a wireless power receiver ; and a power conversion unit configured to transmit a wireless power to the wireless power receiver by generating magnetic coupling in a primary coil according to the first available power indicator, wherein the wireless power transmitter is configured to: receive, from the wireless power receiver , a received power packet indicating a power value received by the wireless power receiver , transmit, to the wireless power receiver during a power transfer phase in which the wireless power is transmitted, a bit pattern response requesting a communication to change the first available power indicator in response to the received power packet, receive, from the wireless power receiver , a response signal indicating being ready to receive the communication, and transmit, to the wireless power receiver during the power transfer phase, a packet related to a second available power indicator after receiving the response signal, wherein the bit pattern response is composed of 8 bits, and wherein the bit pattern response is defined as a different pattern from a bit pattern for an 8-bit acknowledge, ACK, response, from a bit pattern for an 8-bit not acknowledge, NAK, response, and from a bit pattern for an 8-bit ND response. a communication/control unit configured to perform a negotiation for a first available power indicator with a wireless power receiver ; and a power conversion unit configured to transmit a wireless power to the wireless power receiver by generating magnetic coupling in a primary coil according to the first available power indicator, wherein the wireless power transmitter is configured to: receive, from the wireless power receiver , a received power packet indicating a power value received by the wireless power receiver , transmit, to the wireless power receiver during a power transfer phase in which the wireless power is transmitted, a bit pattern response requesting a communication to change the first available power indicator in response to the received power packet, receive, from the wireless power receiver , a response signal indicating being ready to receive the communication, and transmit, to the wireless power receiver during the power transfer phase, a packet related to a second available power indicator after receiving the response signal, wherein the bit pattern response is composed of 8 bits, and wherein the bit pattern response is defined as a different pattern from a bit pattern for an 8-bit acknowledge, ACK, response, from a bit pattern for an 8-bit not acknowledge, NAK, response, and from a bit pattern for an 8-bit ND response. receive, from the wireless power receiver , a received power packet indicating a power value received by the wireless power receiver , transmit, to the wireless power receiver during a power transfer phase in which the wireless power is transmitted, a bit pattern response requesting a communication to change the first available power indicator in response to the received power packet, receive, from the wireless power receiver , a response signal indicating being ready to receive the communication, and transmit, to the wireless power receiver during the power transfer phase, a packet related to a second available power indicator after receiving the response signal, wherein the bit pattern response is composed of 8 bits, and wherein the bit pattern response is defined as a different pattern from a bit pattern for an 8-bit acknowledge, ACK, response, from a bit pattern for an 8-bit not acknowledge, NAK, response, and from a bit pattern for an 8-bit ND response. <EOS>