To prevent a metal belt from vibrating in the course of passing of the metal strip through an air cooling zone.

A pair of or a plurality of pairs of rolls 1 restricting both edge parts of the metal strip 2 are arranged in the air cooling zone. The pair of or the plurality of pairs of rolls 1 are independently held every each roll 1 and a V-shaped groove is formed between paired rolls 1 by bringing into contact with the edge part of the metal strip 2 with inclined angles of the respective rolls 1. Then, the vibration of the metal strip 2 is prevented by restricting both the edge parts of the metal strip 2 to the V-shaped groove without receiving the influence of the displacement in a direction of the metal strip 2. Therefore, the coating irregularity is suppressed to minimum.