A shaft member (32) has the same polarity as discharge electrodes (60). Each of the discharge electrodes (60) has an electrode body (65) and a tip end (63) which is formed at an end of the electrode body (65) facing an inner peripheral surface (21) of a counter electrode (20). The discharge electrodes (60) extend such that the tip ends (63) and the electrode bodies (65) are misaligned with each other in an axial direction of the shaft body (32). A streamer discharge travels from the tip ends (63) of the discharge electrodes (60) toward the inner peripheral surface (21) of the counter electrode (20).
