To stably treat a long substrate to be treated by introducing a treating gas at a prescribed flow rate only into the space between a pair of opposed electrodes whose opposed surfaces are convex.

The surfaces of a pair of fixed electrodes 2, 3 arranged opposite to each other in the vertical direction are coated with a fixed dielectric substance 7, 8 and a discharge space 4 is formed between them. A gas supply nozzle 5 and a gas exhaust nozzle 6 are disposed on both sides of the discharge space and a treating gas is blown from the outlet of the gas supply nozzle 5 into the discharge space 4 in an arrow direction at a prescribed rate (preferably about 1-50 m/sec, more preferably 2-20 m/sec). When an electric field is applied from a power source 1 between the electrodes 2, 3, the treating gas is converted to plasma in the discharge space 4 to treat the surface of a substrate 10 to be treated during transport between the electrodes 2, 3.