One discharge tube assembled from divided three sections is constructed by connecting between a first discharge tube element of metal material and a second discharge tube element of metal material with an insulator for forming a discharging gap and vacuum-sealing interposed between connecting flanges of both discharge tube elements in a state of sealing hermetically with an O-ring and also by connecting between a first discharge tube element of metal material and a third discharge tube element of metal material in a similar manner, and a discharge gap is formed between adjacent discharge tube elements. A high-frequency power supply is connected to each discharge tube element. Respective discharge tube elements are provided with cooling medium flow paths separately each other.