In this paper, a wireless power transmission system using magnetic resonance coupling was proposed and demonstrated for supplying power at high efficiency to electrical devices in a space enclosed by metal walls. This is applicable to power supply to electrical sensors or devices working in the area surrounded by metal walls. Proposed magnetic resonance coupling system is driven at a resonance frequency of 50 Hz, which is selected to avoid eddy current loss on the surrounding metals. Firstly, resonator designs and its performance limitation were described. Secondly, the equivalent circuits and theoretical transmission efficiency were presented. Finally, power transmission was experimentally demonstrated and transmission efficiency was measured in some conceivable situations. As a result, electric power of 3 W was supplied to LEDs over a stainless steel wall. When the stainless steel wall thickness was 10 mm, transmission efficiency of approximately 40% was achieved over the transmission distance of 12 cm. Moreover, in the demonstration of transmission through a metal pipe, 1.2 W of power was transmitted to LEDs in a 10 mm thick metal pipe.
 T. Kuroda, “Advances in VLSI Technologies for UltraLow-Power Computing: Creation of High Performance Ultra-Low-Power Short-Range Wireless Mobile Information Systems,” Information Processing, Vol. 51, No. 7, 2010, pp. 861-869 (in Japanese)
 A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher and M. Soljacic, “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” Science, Vol. 317, No. 5834, 2007, pp. 83-86.
 A. Karalis, J. D. Joannopoulos and M. Soljacic, “Efficient Wireless Non-Radiative Mid-Range Energy Transfer,” Annals of Physics, Vol. 323, No. 1, 2008, pp. 34-48.
 T. Imura, H. Okabe and Y. Hori, “Basic Experimental Study on Helical Antennas of Wireless Power Transfer for Electric Vehicles by Using Magnetic Resonant Couplings,” 2009 IEEE Vehicle Power and Propulsion Conference (VPPC 2009), Dearborn, 7-10 September 2009, pp. 936-940. http://dx.doi.org/10.1109/VPPC.2009.5289747
 T. Komaru, M. Koizumi, K. Komurasaki, T. Shibata and K. Kano, “Compact and Tunable Transmitter and Receiver for Magnetic Resonance Power Transmission to Mobile Objects,” In: K. Y. Kim, Ed., Wireless Power Transfer—Principles and Engineering Explorations, InTech, Rijeka, 2012, pp. 133-150.
 Fei Zhang, Xiaoyu Liu, S. A. Hackworth, R. J. Sclabassi and Mingui Sun, “In Vitro and in Vivo Studies on Wireless Powering of Medical Sensors and Implantable Devices,” 2009 IEEE/NIH Life Science Systems and Applications Workshop (LiSSA 2009), Bethesda, 9-10 April 2009, pp. 84-87.
 Sanghoek Kim, J. S. Ho, L. Y. Chen and A. S. Y. Poon, “Wireless Power Transfer to a Cardiac Implant,” Applied Physics Letters, Vol. 101, No. 7, 2012, pp. 073701-073701-4. http://dx.doi.org/10.1063/1.4745600
 H. Zangl, A. Fuchs, T. Bretterklieber, M. J. Moser and G. Holler, “Wireless Communication and Power Supply Strategy for Sensor Application within Closed Metal Walls,” IEEE Transactions on Instrumentation and Measurement, Vol. 59, No. 6, 2010, pp. 1686-1692.
 H. Zangle, A. Fuchs, T. Bretterklieber, M. J. Moser and G. Holler, “An Investigation on Wireless Communication and Power Supply through Metal Tank Walls,” 2008 IEEE Instrumentation and Measurement Technology Conference Proceedings (IMTC 2008), Victoria, 12-15 May 2008, pp.1452-1457
 T. Komaru, K. Komurasaki, M. Koizumi, T. Shibata and K. Kano, “Parametric Evaluation of Mid-Range Wireless Power Transmission,” 2010 IEEE International Conference on Industrial Technology (ICIT 2010), Vina der Mar, 14-17 March 2010, pp. 789-792.
 M. Koizumi, K. Komurasaki, Y. Mizuno and Y. Arakawa, “Wireless Power Feeding with Strongly Coupled Magnetic Resonance for a Flying Object,” Wireless Engineering and Technology, Vol. 3, No. 2, 2012, pp. 86-89.