A lateral flow atomic layer deposition device according to an exemplary embodiment of the present invention eliminates a gas flow control plate in a conventional lateral flow atomic layer deposition device and controls shapes of a gas input part and a gas output part in a reactor cover to make a gas flow path to a center of a substrate shorter than a gas flow path to an edge of the substrate and thereby increase the amount of gas per unit area flowing to the center of the substrate. Therefore, film thickness in the center of the substrate in the lateral flow reactor increases.