A back massaging device includes two eccentric rotation members each having an eccentric hole and a resting face, and two swing arms each provided with a mounting hole which has a sliding face. Thus, when each of the swing arms is pressed by the user's body, the sliding face of each of the swing arms is slidable on the resting face of the respective eccentric rotation member to freely adjust the angle of each of the swing arms, so that the first massaging member and the second massaging member of each of the swing arms touch the user's body evenly and exactly, and the pressing force applied by the first massaging member and the second massaging member are distributed evenly and smoothly on the user's body.