This plasma generator has a reaction chamber 140, which is divided into a pair of insertion parts 106 for inserting a pair of electrodes and a coupling part 108 for coupling the pair of insertion parts. The coupling part is narrower than the pair of insertion parts. Consequently, discharge occurs in the entire region of the coupling part, and from the insertion parts along the wall surface continuous with the coupling part. Also, the coupling part being narrower than the insertion parts reduces the volume of the coupling part. Consequently, a treatment gas undergoes plasma transformation efficiently by a small volume of treatment gas undergoing plasma transformation in the entire region of the coupling part and from the insertion parts along the wall surface continuous with the coupling part. Also, the coupling location of the coupling part with each of the pair of insertion parts is a smooth surface 110. Discharge-induced heat is thereby dispersed to the entire smooth surface, and discharged-induced carbonization is suppressed.