I. Introduction
In recent years, wireless power transfer methods have been attracted in the community [1-2]. In
particular, the wireless power transfer with magnetic resonance coupling (MRC) which is reported by
A. Karalis et al. in 2007 is heavily studied [3-4]. The MRC shows better features compared with the
conventional wireless power transfer methods such as an electromagnetic induction and a microwave
transmission. First, the MRC allows a wireless power transfer in a middle-range transmission distance
such as a few dozen of centimeters at high efficiency of over 90% [5]. Such high transmission
efficiency cannot be achieved with an electromagnetic induction. On the other hand, the transmission
efficiency of a microwave transmission is greater than the others. However, the conversion efficiency
between microwaves from electrical power is mere 84.4% [6].