We propose a magnetic field resonance type wireless power transfer system for the Li-ion battery (Lithium-ion battery). The transmitting efficiency is changed by the changing of the charging current to the storage battery. The system places the DC/DC converter at both of the primary and secondary sides. This system suppresses the degradation of transmitting efficiency due to the secondary side load variation by controlling the primary side DC/DC. By experiments, this system suppressed only 4.4% degradation of transmitting efficiency. It is significant improvement from 16% degradation by conventional approaches. Thus, this system derives both of optimum charging to storage battery and improvement of the transmission efficiency for wide range of output current.