A medium transportation device includes a transportation driving roller that transports a medium; a roller unit including a separation roller as a driven roller that is rotated by rotation of the transportation driving roller and a shaft which penetrates a rotation center of the separation roller; a holder that detachably holds the roller unit; and a lock mechanism that switches between an unlocked state in which the roller unit is unlocked from the holder and a locked state in which the roller unit is locked to the holder.