PURPOSE: To improve the working rate of the device by constituting the device in such a manner that gases are released into a base body treating space through a porous structural body.

CONSTITUTION: The structural body 2 consisting of a porous dielectric substance is arranged so as to separate a treatment chamber 6 and a microwave introducing window 1 on the vacuum side near this microwave introducing window. A shielding member 3 for preventing the arrival of the active gaseous species formed in the treatment chamber 6 is arranged on the surface of the structural body 2 adjacent to this structural body 2. A gas introducing line 5 for introducing the plasma forming gas is disposed in the closed space constituted of at least the structural body 2 and the microwave introducing window 1. The treatment device is so constituted that the gases are released into the base body treatment space by passing the structural body 2 in such a manner. As a result, the production stability is improved and the breakdown of the hermeticity of the reaction chamber is prevented.