PURPOSE: To improve the accuracy of measurement by providing a pair of measuring contactors which are kept applied with measuring pressures in opposite directions on measuring shafts.

CONSTITUTION: An outside casing 11 of a cylindrical shape has holes 12A, 12B centering at a measuring shaft in both end parts, and has a connecting rod 13 projecting from the central part in the longitudinal direction. A pair of measuring contactors 14A, 14B provided in the casing 11 are guided by the holes 12A, 12B and move the measuring shafts mutually independently. Coaxial type magnetic scale devices 16A, 16B consisting of rod-like magnetic scales 17A, 17B and magnetic detecting heads 18A, 18B are provided at the trailing ends of the contactors 14A, 14B. The contactors 14A, 14B and the scales 17A, 17B are coaxially fixed, and further coil springs 20A, 20B that exert prescribed measuring pressures upon the contactors 14A, 14B are disposed between the collars 21A, 21B fixed to the measuring contactors and the collars 22A, 22B of the magnetic heads.