A wet electrostatic precipitator for removing SO 3 and dust contained in gas, comprising: an electric field forming unit which includes a first electrode (20, 30) and a second electrode (21, 31) that are arranged to oppose each other along a flow direction of the gas containing mist having the SO 3 incorporated therein and the dust so as to form a direct current electric field, wherein the first electrode is a flat plate and includes a plurality of discharge electrodes (23, 33) formed on a surface of the first electrode that opposes the second electrode, along the flow direction of the gas at predetermined intervals, the second electrode includes a discharge frame (24, 34), a first flat plate portion (25a, 35a) which extends in a direction substantially perpendicular to the flow direction of the gas and is provided at a position that opposes the discharge electrode of the first electrode, and a second flat plate portion (25b, 35b) which extends in the direction substantially perpendicular to the flow direction of the gas, is provided separated from the first plate portion, and has a plurality of discharge electrodes (23, 33) formed on a surface that opposes a flat surface part of the first electrode, the first flat plate portion and the second flat plate portion are arranged along the flow direction of the gas, the discharge electrode of the first electrode and the discharge electrode of the second electrode are arranged to alternately generate corona discharges having opposite polarities in the direction perpendicular to the flow direction of the gas so as to alternately apply charges having opposite polarities to the mist and the dust by the corona discharges when the gas passes through between the first electrode and the second electrode, and the first electrode and the first flat plate portion are arranged to trap the charged mist and the dust. an electric field forming unit which includes a first electrode (20, 30) and a second electrode (21, 31) that are arranged to oppose each other along a flow direction of the gas containing mist having the SO 3 incorporated therein and the dust so as to form a direct current electric field, wherein the first electrode is a flat plate and includes a plurality of discharge electrodes (23, 33) formed on a surface of the first electrode that opposes the second electrode, along the flow direction of the gas at predetermined intervals, the second electrode includes a discharge frame (24, 34), a first flat plate portion (25a, 35a) which extends in a direction substantially perpendicular to the flow direction of the gas and is provided at a position that opposes the discharge electrode of the first electrode, and a second flat plate portion (25b, 35b) which extends in the direction substantially perpendicular to the flow direction of the gas, is provided separated from the first plate portion, and has a plurality of discharge electrodes (23, 33) formed on a surface that opposes a flat surface part of the first electrode, the first flat plate portion and the second flat plate portion are arranged along the flow direction of the gas, the discharge electrode of the first electrode and the discharge electrode of the second electrode are arranged to alternately generate corona discharges having opposite polarities in the direction perpendicular to the flow direction of the gas so as to alternately apply charges having opposite polarities to the mist and the dust by the corona discharges when the gas passes through between the first electrode and the second electrode, and the first electrode and the first flat plate portion are arranged to trap the charged mist and the dust.