The voltage application unit first applies voltages (+,+,0,-,-,0) respectively to the electrostatic delivery electrodes 37 belonging to the phase `a`, the phase `b`, the phase `c`, the phase `d`, the phase `e`, and the phase `f`, and then successively applies voltages (0,+,+,0,-,-), voltages (-,0,+,+,0,-), voltages (-,-,0,+,+,0), voltages (0,-,-,0,+,+), and voltages (+,0,-,-,0,+). The voltage application unit repeats this cycle multiple times to apply the voltages to the phase `a` through the phase `f`. The water droplets flocculated in the oxidizing gas conduits 36 are charged by electrostatic induction and travel in the direction from the inlet to the outlet of the oxidizing gas conduits 36 while being repelled or attracted by the electrostatic delivery electrodes 37 in the vicinity of the water droplets in the course of the positive-negative variation of the voltage in the cycle.