A shaft member has the same polarity as discharge electrodes. Each of the discharge electrodes has an electrode body and a tip end which is formed at an end of the electrode body facing an inner peripheral surface of a counter electrode. The discharge electrodes extend such that the tip ends and the electrode bodies are misaligned with each other in an axial direction of the shaft body. A streamer discharge travels from the tip ends of the discharge electrodes toward the inner peripheral surface of the counter electrode.
