A processing gas diffusing and supplying unit is provided in a substrate processing unit including a processing chamber for accommodating a substrate. The processing gas diffusing and supplying unit comprises a main body; a plate supported by the main body and having a plurality of gas supply holes; a partition wall; an internal space having a first and a second space partitioned by the partition wall; a first and a second opening respectively communicating with the first and the second space while facing the plate, first and a second space being connected to a first and a second processing gas introducing pipe of the processing chamber, respectively; and a first and a second shielding portion respectively installed in the first and the second space and having a surface facing the first and the second opening.