In a magnetic tape recording/playing device, a reel base driving mechanism wherein a pulley driven by an electric motor and a reel drive block are fitted rotatably independently of each other to a shaft of each of forward- and reverse-side reel bases, while an idler is movably provided between said reel bases, which idler having an input section or idler which engages said pulleys and an output section or idler which engages said reel drive blocks, said idler being made to engage the forward- or reverse-side reel base, thus transmitting the rotation of the pulley on that reel base to the reel drive block also on this reel base.