In a composite magnetic body comprising soft magnetic powder dispersed in an organic binding agent and having an electromagnetic interference suppressing effect, powder excellent in thermal conductivity is further dispersed into the organic binding agent so as to provide a composite magnetic body excellent in thermal conductivity. This composite magnetic body may also be used as a heat dissipation sheet for an electronic device. Further, it may also constitute a heat sink having an electromagnetic interference suppressing effect. As the organic binding agent, thermoplastic polyimide or liquid crystal polymer can be cited. As the powder excellent in thermal conductivity, Al2O3, AlN, cubic BN, insulating SiC or a heat conductive reinforcement (capton) can be cited.