The effective thermal conductivities of SiN and TbFeCo thin films were measured by comparing the length and width of polarizing microscope image of thermo-magnetically written domains with those of calculated isotherms for the trilayer structure of substrate/Si3N4/Tb22Fe70C08/Si3N4. The resulting data were applied to the quadrillayer structure of substrate/Si3N4/Tb22Fe70C08/Si3N4/Al, and the length of calculated isotherms was turned out to agree with that of written domain.