Disclosed is an apparatus for distributing gas which is capable of uniformly injecting processing gas into a plurality of gas passages being communicated with a plurality of gas distribution holes, and an apparatus for processing substrate including the same, wherein the apparatus for distributing gas may include a body including a plurality of gas passages connected with a plurality of gas distribution holes for distributing processing gas; and at least one gas injection module connected with at least one lateral surface of the body and respectively communicated with the plurality of gas passages, wherein the gas injection module firstly buffers the processing gas supplied from the external, secondly buffers the firstly buffered processing gas, and injects the buffered processing gas into the plurality of gas passages.