It is a very serious problem for an implantable artificial heart to receive energy from outside the body. A new implantable transformer with a plain weave structure was proposed by using amorphous magnetic fibers. The transformer has the following features: (i) a thin, planar shape; (ii) high flexibility; and (iii) low waste heat in the range of several tens kHz. Appropriate conditions for the size of the transformer were shown experimentally. The transformer (70×30×1 mm3) of a trial design can supply a power of 6 W. The temperature rise of the transformer based on iron loss is considered to be negligibly small.