PURPOSE: To always control the tension of a magnetic tape in accordance with a winding diameter of a take-up reel including the start-up time by eliminating the noncontroled traveling due to a long period of detecting a winding diameter at the starting time of traveling the magnetic tape.

CONSTITUTION: The device is constituted of a tension control circuit equipped with a winding diameter detecting circuit 7 for controlling a driving circuit 6 for a reel motor 1 to drive the reel 2 for taking up the magnetic tape 3 according to the winding diameter and a detecting period changeover circuit 8 for setting the winding diameter detecting period of the winding diameter detecting circuit 7 to be short at the time of start and also setting it to be varied longer with the elapse of time.