Communication system using wireless power

Provided is a device and system for performing communication using wireless power, and a communication device using wireless power. The device may calculate a slope of an envelope in a waveform of energy stored in a target resonator through mutual resonance with a source resonator, and 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.