Heart side magnets 20 and 21 are fixedly secured to the surface of a heart 100. A material which has a higher rigidity than the cardiac muscle is molded to form a molded member 30. Molded member side magnets 40 and 41 are attached to the molded member 30 so as to correspond to the heart side magnets 20 and 21. The polarity of one of the heart side magnets 20 and 21 and the molded member side magnets 40 and 41 is switched at predetermined timings, whereby the heart 100 is magnetically expanded and contracted to improve the pumping function.