Abstract: This invention relates to tape tension control in a device which has a capstan disposed between a supply reel and which transport a tape wound around the supply reel to the take-up reel. While the tape, mounted on the reels, is running, the tape speed is varied greatly, and during this speed change period, a tape tension variation between the supply reel and the capstan and a tape tension variation between the capstan and the take-up reel are found. From those tape tension variations, ratios of moments of inertia of the reels with the tape thereon to the tape radii are found, and according to the values of the ratios, the gains of the loops of the tension control system is adjusted.