The invention provides a magnet unit and a magnetron sputtering device. Disclosed is a magnet unit which can homogenize the thickness distribution of a thin film to be formed on a substrate, without increasing the length and width of a target. The magnet unit (10) comprises an annular outer magnet (30) arranged over a yoke (20) on the back side of a cathode electrode and along the contour of a target (6), and an inner magnet (40) arranged in the annular outer magnet and made different in polarity from the outer magnet, whereby the tangents of magnetic lines of force (M) generated over the target form the magnetic tracks (MT) of a set of areas parallel to the target face. The magnet unit (10) also comprises extending magnetic poles (41) of an n-number (n indicates a positive integer of 2 or more) extending from the central portion of the inner magnet and approaching the two longitudinal ends of the outer magnet, and protruding magnetic poles (32) of a (n-1)-number protruding longitudinally inward from the inner sides of the two ends of the outer magnet and positioned between the extending magnetic poles of the n-number. The extending magnetic poles of the n-number and the protruding magnetic poles of the (n-1)-number form folded-back sections (U) of a (2n-1)-number at the two longitudinal ends of the magnetic track.