A thin film magnetic sensor comprises a pair of thin film yokes each formed of a soft magnetic material, the thin film yokes being arranged to face each other with a gap interposed therebetween; a GMR film electrically connected to the pair of the thin film yokes and having an electrical resistivity higher than that of the soft magnetic material; and an insulating substrate supporting the thin film yokes and the GMR film and formed of an insulating nonmagnetic material. A gap column of a multilayer structure including a layer formed of an insulating nonmagnetic material and a layer of the GMR film is arranged within the gap, and the thickness of the GMR film is uniform over the gap length.