An exhaust gas treatment apparatus comprising a scrubber for an engine or boiler comprising: a preprocessor configured to primarily remove harmful substances from exhaust gas produced by combustion; and a postprocessor configured to further remove harmful substances from preprocessed gas, which is the exhaust gas from which the harmful substances have been primarily removed by the preprocessor, wherein the postprocessor comprises: a postprocessor housing having a preprocessed gas inlet through which the preprocessed gas is introduced and a postprocessed gas outlet through which postprocessed gas from which harmful substances have been further removed by the postprocessor is discharged and forming a flow path of the preprocessed gas therein; and a diffuser disposed adjacent to and in front of the preprocessed gas inlet so as to be spaced apart therefrom in the postprocessor housing and configured to diffuse the preprocessed gas introduced through the preprocessed gas inlet to the inside of the postprocessor housing, and wherein the diffuser is disposed to cover the front of the preprocessed gas inlet and comprises a body having a diffusion part through which the preprocessed gas passes, characterized in that the body is formed so as to vertically cover the front of the preprocessed gas inlet, that upper and lower ends of the body are inclined or curved upwards and downwards, respectively, toward the preprocessed gas inlet, and that two or more diffusers are arranged in sequence in front of the preprocessed gas inlet. a preprocessor configured to primarily remove harmful substances from exhaust gas produced by combustion; and a postprocessor configured to further remove harmful substances from preprocessed gas, which is the exhaust gas from which the harmful substances have been primarily removed by the preprocessor, wherein the postprocessor comprises: a postprocessor housing having a preprocessed gas inlet through which the preprocessed gas is introduced and a postprocessed gas outlet through which postprocessed gas from which harmful substances have been further removed by the postprocessor is discharged and forming a flow path of the preprocessed gas therein; and a diffuser disposed adjacent to and in front of the preprocessed gas inlet so as to be spaced apart therefrom in the postprocessor housing and configured to diffuse the preprocessed gas introduced through the preprocessed gas inlet to the inside of the postprocessor housing, and wherein the diffuser is disposed to cover the front of the preprocessed gas inlet and comprises a body having a diffusion part through which the preprocessed gas passes, characterized in that the body is formed so as to vertically cover the front of the preprocessed gas inlet, that upper and lower ends of the body are inclined or curved upwards and downwards, respectively, toward the preprocessed gas inlet, and that two or more diffusers are arranged in sequence in front of the preprocessed gas inlet. a postprocessor housing having a preprocessed gas inlet through which the preprocessed gas is introduced and a postprocessed gas outlet through which postprocessed gas from which harmful substances have been further removed by the postprocessor is discharged and forming a flow path of the preprocessed gas therein; and a diffuser disposed adjacent to and in front of the preprocessed gas inlet so as to be spaced apart therefrom in the postprocessor housing and configured to diffuse the preprocessed gas introduced through the preprocessed gas inlet to the inside of the postprocessor housing, and wherein the diffuser is disposed to cover the front of the preprocessed gas inlet and comprises a body having a diffusion part through which the preprocessed gas passes, characterized in that the body is formed so as to vertically cover the front of the preprocessed gas inlet, that upper and lower ends of the body are inclined or curved upwards and downwards, respectively, toward the preprocessed gas inlet, and that two or more diffusers are arranged in sequence in front of the preprocessed gas inlet.