A synchronizing device includes a pair of longitudinal guiding units each having a rack member and a movement damper connected to the rack member, and a rotating mechanism including a pair of pinion gears to be meshed respectively with the guiding units. When the pinion gears move respectively from the rack members for rotation respectively on the movement dampers, an increased pressure is produced between the guiding units and the rotating mechanism, thereby slowing down and damping the rotation of the rotating mechanism.