Small implantable medical devices, such as wireless capsule endoscopes, that can be swallowed have previously been developed. However, these devices cannot continuously operate for more than 8 h because of battery limitations; moreover, additional functionalities cannot be introduced. This paper proposes a design method for a high-efficiency energy transmission transformer (ETT) that can transmit energy transcutaneously to small implantable medical devices using electromagnetic induction. First, the authors propose an unconventional design method to develop such a high-efficiency ETT. This method can be readily used to calculate the exact transmission efficiency for changes in the material and design parameters (i.e., the magnetic material, transmission frequency, load resistance, etc.). Next, the ac-to-ac energy transmission efficiency is calculated and compared with experimental measurements. Then, suitable conditions for practical transmission are identified. A maximum efficiency of 33.1% can be obtained at a transmission frequency of 500 kHz and a receiving power of 100 mW for a receiving coil size of ¿5 mm × 20 mm. Future design optimization is possible by using this method.