This paper describes the output characteristics of a new transformer for an implantable artificial heart. The implantable transformer consists of Co-base amorphous magnetic fiber and conductive fibers. Features include a thinf planar shape and high flexibility. A trial-fabricated transformer can supply an electric power of 10 W. The magnetic field distribution in the vicinity of the transformer was measured, and it was possible to reduce the leakage flux behind it. A transformer with a spiral woven structure is also discussed.