A communication device using wireless power, the communication device being configured to communicate with a transmitter that comprises a source resonator to transmit power and data wirelessly, said transmitter having a switch to either store energy from a power supply in the source resonator or to electrically disconnect the power supply from the source resonator thereby inducing self-resonance of the source resonator, wherein the transmitter is configured to operate the switch in accordance with an on and off timing, said communication between the transmitter and the communication device being based on transmitter cycles, wherein, in each transmitter cycle, energy is first stored in the source resonator and is then transmitted to the communication device through mutual resonance, the device comprising: a receiver comprising a target resonator for receiving energy transmitted from the source resonator of the transmitter to the target resonator through mutual resonance, wherein the receiver comprises a switch to either disconnect a load in order to charge the source resonator through the mutual resonance or to connect said load in order to terminate the mutual resonance by changing a resonance frequency of the target resonator with respect to a resonance frequency of the source resonator; an envelope detector configured to detect an envelope in a waveform of a current or a voltage applied to the target resonator; a calculator configured to calculate a slope of the envelope; an estimation unit configured to estimate a point placed within a predetermined interval around a point at which the slope reaches a maximum as a starting point of the mutual resonance between the source resonator and the target resonator; wherein the communication device is configured to: before having established time synchronization, maintain the switch of the receiver in an off state to charge the target resonator, to accumulate, during a predetermined period, a signal that is applied to the target resonator through mutual resonance in response to energy being transmitted by the transmitter, to detect an envelope during a time synchronization period based on the accumulated signal, and to estimate a starting point of mutual resonance for a subsequent transmitter cycle, said time synchronization period comprising a plurality of transmitter cycles; and after having established time synchronization, to receive the data from the transmitter during a processing duration by turning the switch off at the estimated starting point thereby enabling mutual resonance. a receiver comprising a target resonator for receiving energy transmitted from the source resonator of the transmitter to the target resonator through mutual resonance, wherein the receiver comprises a switch to either disconnect a load in order to charge the source resonator through the mutual resonance or to connect said load in order to terminate the mutual resonance by changing a resonance frequency of the target resonator with respect to a resonance frequency of the source resonator; an envelope detector configured to detect an envelope in a waveform of a current or a voltage applied to the target resonator; a calculator configured to calculate a slope of the envelope; an estimation unit configured to estimate a point placed within a predetermined interval around a point at which the slope reaches a maximum as a starting point of the mutual resonance between the source resonator and the target resonator; wherein the communication device is configured to: before having established time synchronization, maintain the switch of the receiver in an off state to charge the target resonator, to accumulate, during a predetermined period, a signal that is applied to the target resonator through mutual resonance in response to energy being transmitted by the transmitter, to detect an envelope during a time synchronization period based on the accumulated signal, and to estimate a starting point of mutual resonance for a subsequent transmitter cycle, said time synchronization period comprising a plurality of transmitter cycles; and after having established time synchronization, to receive the data from the transmitter during a processing duration by turning the switch off at the estimated starting point thereby enabling mutual resonance. before having established time synchronization, maintain the switch of the receiver in an off state to charge the target resonator, to accumulate, during a predetermined period, a signal that is applied to the target resonator through mutual resonance in response to energy being transmitted by the transmitter, to detect an envelope during a time synchronization period based on the accumulated signal, and to estimate a starting point of mutual resonance for a subsequent transmitter cycle, said time synchronization period comprising a plurality of transmitter cycles; and after having established time synchronization, to receive the data from the transmitter during a processing duration by turning the switch off at the estimated starting point thereby enabling mutual resonance.