An apparatus for plating a metal strip in an electrolytic cell is provided with a supporting roll. The support roll is disposed apart from the deflector rolls by a predetermined distance and supports the metal strip in such a way that the catenary of the metal strip can be reduced. The distance is determined so that the catenary is 0.04 or less times of the distance between upper and lower electrodes in the electrolytic cell. When the metal strip is a steel plate having a density of 7.85.times.10.sup.-6 (kg/mm.sup.3), the predetermined distance l (mm) between the axis of the deflector roll and that of the supporting roll is subject to the following formula: ##EQU1## in which the distance between the upper and lower electrodes is .DELTA.D (mm), the thickness of the metal strip being t (mm), the width thereof being W (mm) and the tension applied to the metal strip being T (kg).