The thin film magnetic sensor comprising a GMR film having a Giant Magneto-Resistance effect; and thin film yokes formed of a soft magnetic material connected electrically to both ends of the GMR film; wherein the thin film yoke has a high sensitivity portion with a demagnetizing factor of N.sub.L in a magnetic sensitive direction, and a low sensitivity portion with a demagnetizing factor of N.sub.H(>N.sub.L) in the magnetic sensitive direction, the low sensitivity portion being connected electrically in series with the high sensitivity portion.