Yosuke DAICHO (Stu.Mem.), Yasuhiro KAWASHIMA (Stu. Mem.), Yinggang BU (Mem.), Tutomu MIZUNO (Mem.), Ryosuke MORIKAWA The wireless power transfer is a technique to send electricity without having the mechanical point of contact. The wireless power transfer using magnetic resonant coupling has been studied. The charge to a cell-phone has already been possible by Wireless power transfer. Qi standards are used for wireless power transfer well. The output is less than 5 W. Frequency to use is 110-205 kHz. A wireless power transfer system having the output about 40 W will be necessary in future to charge an electric tool. In this paper, we performed wireless power transfer of output electricity 40 W using the magnetic resonant coupling. Transmission efficiency at 40 W transmission was measured. Also, we measured the radiation electromagnetic field at 40 W transmission. As a result, the efficiency between the coils achieved 91.7 %. The radiation electromagnetic field was less than guidelines on ICNIRP.Keywords: wireless power transfer, magnetic resonant coupling, Qi, radiation electromagnetic field, ICNIRP.