PURPOSE: To obtain ionized gas high in ion concn. under a relatively low temp. state, by flowing gas to be ionized containing at least steam as a component into a region in which at least either one of an electric field and an electromagnetic wave field is formed to ionize the same.

CONSTITUTION: Gas to be ionized comprising steam or gas to be ionized obtained by mixing gas containing a minute amount of an ion and steam is flowed into a region 32 in which an electric field or an electromagnetic wave acted field is formed and ionized by utilizing the thermal movement of steam molecules high dielectric property of the moisture and the amplifying action of collision ionization generated during the movement of gas to be ionized through the aforementioned region. By this method, the ionized gas high in ion concn. can be obtained under a relatively low temp. state and this method can largely contribute to the enhancement of the effectiveness in various fields utilizing the ionized gas such as a M.H.D. power generating appartus.