A magnetic circuit for a voice coil motor of flat-coil type used with a magnetic device is disclosed. The magnetic circuit is intended to flatten the torque constant curve for swinging the rotor of the voice coil motor while at the same time preventing the torque constant from decreasing. For this purpose, the magnetic circuit comprises an upper yoke and a lower yoke opposed to a flat coil arranged on the rotor, a pair of side yokes for coupling the upper and lower yokes to each other at the ends thereof, and a magnet arranged on the lower yoke. The magnet is shaped arcuate so that the width in the neighborhood of each end thereof in the direction of motion of the flat coil is larger than a standard width and smaller than the standard width in the portions of the magnet between each end and the central portion thereof.