In a measurement mechanism for continuously measuring a thickness of a coating layer, provided in an apparatus for forming the coating layer on a conductive elongate base material in a coating treatment base station while the base material is fed, a sensing portion for measuring a capacitance value of the coating layer is arranged before and after the base station, and tension applied to the base material at the sensing portion is set to be greater than tension applied to the base material at the base station. Thus, in forming the coating layer on the elongate base material while the base material is continuously fed, variation in a feeding speed is suppressed, influence of sway of a measurement surface in a direction of thickness at the thickness sensing portion during feeding is minimized, and a thickness of the coating layer can be measured with higher accuracy.