The present invention is directed to a magnetic thin-film device whose operating frequency band ranges from several millihertz (MHz) to several gigahertz (GHz) and which is used as an inductor for a switching power supply, a noise filter, a reception circuit for receiving a quasi-microwave and a magnetic sensor. In this device, uniaxial magnetic anisotropy is guided to a magnetic layer, and the magnetic layer is sandwiched between dielectric layers to form a propagation path of electromagnetic wave. A microstrip line is provided on the top surface of the propagation path, while an insulative underlying substrate is formed on the bottom surface thereof with a lower grounded conductor interposed therebetween. Thus, the wavelength of the propagation path can be shortened to miniaturize the device. The device is rapidly improved in characteristics and miniaturized further, resulting in reduction in manufacturing costs.