An apparatus for treating sewage using a semi-batch process is provided. The apparatus may include an aeration tank, a thickening tank, a dephosphorization tank, and first and second reactor basins. The first and second reactor basins may each include an opening portion formed by opening a side wall thereof so as to be connected with a lateral portion of the aeration tank and an aeration unit installed therein. The apparatus may also include a flow distribution pipe including a start pipe and a pair of flow pipes, a pair of stirrers each provided at a respective side of the auxiliary pipes in each of the pair of flow pipes, operating in turn to stir the dephosphorized sewage overflowing from the dephosphorization, and a decanter installed across the top of the side opposite to the opening portions of the first and second reactor basins.