PURPOSE: To efficiently enable switching of precision rolling for adjusting a dimentional shape to a ordinary rolling other than the precision rolling in the operating method of a bar mill train and the rolling mill train.

CONSTITUTION: In the bar mill train, the main body of one rolling mill with which rolling is successively executed after at least one stand 7 for rolling a rolled stock into a circular cross section is made freely changable and the bar mill train in which a 2-roll mill and block type mill which consists of at least two 3-roll mills 8 are changably composed are provided. In the rolling mill train, operation is executed by incorporating the 2-roll mill at the time of precision rolling and the 2-roll mill or the block type mill 8 which consists of at least two 3-roll mills at the time of non-precision rolling.