Contactless power supply systems transmit electric power by electromagnetic induction with a pair of coils. The efficiency and the output voltage depend on parameters of coils. We studied the best parameter values that realize stable high efficiency contactless power supply. In the application with low output voltage, the voltage drop of the diodes becomes dominant power loss. A synchronous rectification was proposed as a solution of this problem. Our contactless power supply system for an artificial heart operates at 190 kHz. We use planar coils with Mn-Zn ferrite cores. Highly stable output voltage and 88% maximum efficiency were realized.