For production of plasma from a medium gas mass in an elongated shape, electric field forming elements 3, 4 that form an electric field in the medium gas mass are provided. The electric field forming elements form an electric field so that partial discharge occurs from the electric field forming elements toward both sides in the longitudinal direction of the medium gas mass. Accordingly, plasma 5 is produced from the medium gas mass. The medium gas mass is formed by, for example, gas supply members 1,2 that guide medium gas, through an internal hollow, to the electric field forming elements. An electric field forming area includes, for example, at least one high-potential electrode 3 and a voltage applying unit 4 that applies a voltage to the high-potential electrode. Plasma limited in medium gas can be produced with high energy efficiency stably over a wide range of parameters through a simple configuration.