To obtain an impact wave by generation of arc discharge between a high-voltage-side electrode 31 connected to a high-voltage-side terminal of a pulse power generating device and a low-voltage-side electrode 32 grounded or connected to a low-voltage-side terminal of the power source. One of the high-voltage-side electrode 31 or the low-voltage-side electrode 32 is an annular electrode formed in an annular shape, the other electrode is a core electrode arranged inside the annular electrode, and arc discharge is generated between an inner peripheral portion of the annular electrode and an outer peripheral portion of the core electrode.