To provide a tension adjusting device for thin wires embodied in a simple constitution capable of adjusting a low tension of the wire accurately.

A dancer roll 28 of this tension adjusting device 20 is borne rotatably by the periphery of a sleeve 34 put on a support table 32. A magnetic field generating coil 38 is coaxially fixed to the inside circumferential surface of the sleeve 34, and a permanent magnet 40 is arranged penetrating the axis of the coil 38. The permanent magnet 40 is a rod-shaped magnet formed so that N-poles and S-poles appear alternately in the longitudinal direction, and its two ends are fixed to stoppers 42 and 44 formed on the support table 32. In this constitution, a magnetic force is produced when current is fed to the coil 38, which generates a thrust force, whereby the coil 38 and the sleeve 34 move in a single piece in the horizontal direction, and the dancer roll 28 borne by the sleeve 34 makes displacement in the horizontal direction.