PURPOSE: To uniformize the distribution of reaction gas on the surface of each specimen by performing evacuation through a central exhaust duct, by providing a central exhaust duct having exhaust ports provided thereto at a position symmetric to a specimen.

CONSTITUTION: A central exhaust duct, wherein a pair of specimen holders 10 capable of determining the positions of specimens 9 at positions opposed to a pair of opposedly arranged electrodes are provided between said electrodes are exhaust ports 5b, 5c are mounted at positions symmetric to the specimens 9 held to the speciment holders 10, is arranged between a pair of the specimen holders 10. By evacuation through this duct 5, reaction gas introduced against the surfaces of the specimens 9 can be exhausted to a symmetric direction without being exhausted to one direction as is conventional and, as a result, the distribution of the reaction gas at the surface of each specimen can be enhanced and the uniformization of distribution can be easily achieved.