A microwave plasma processing device and a gas supply member capable of forming a uniform thin film on a substrate to be processed. The microwave plasma processing device comprises fixing means of fixing a substrate to be processed onto the center axis in a plasma processing chamber, exhaust means of depressurizing the inside and outside of the substrate, a metal processing gas supply member present in the substrate and forming a reentrant cylindrical resonating system along with the plasma processing chamber, and microwave introducing means of introducing a microwave into the plasma processing chamber to process it, wherein a microwave sealing member is provided in a specified position of the substrate-holding portion of the fixing means, and the connection position of the microwave introducing means is set to a specified weak-field position out of a field intensity distribution formed in the interior of the plasma processing chamber.