An operation mode change-over device for a tape recorder has a detection mechanism including a detection member comprised of a fine wire made of a spring material for detecting a stoppage of a reel rotation. The detection member is disposed in elastic contact at its intermediate curved portion and a tip end portion of a V-shaped outer half with the reel shaft. As the reel shaft rotates, the detection member receives rotary torque from the reel shaft to be angularly deviated, so that an end of an inner half of the detection member is engaged with a lever of the change-over device, to thereby apply an acting force to the lever. When the reel shaft stops rotating and the acting force disappears, the lever is brought into engagement with a projection of an idle gear shaft and is urged by the projection with subsequent rotation of the idle gear shaft, to thereby move a slide plate for changeover between forward and reverse modes of the tape recorder.