An antibody or antigen-binding fragment thereof, which binds with a high pM-level affinity to a human IL-4 receptor alpha chain that is a human IL-4 receptor, is provided. The antibody or antigen-binding fragment has a different epitope and a different antigen dissociation rate than existing antibodies. A nucleic acid encoding the antibody or antigen-binding fragment thereof, a vector including the nucleic acid, a cell transformed with the vector, a method for producing the antibody or antigen-binding fragment thereof, a conjugate comprising the antibody or antigen-binding fragment thereof, a composition for preventing or treating inflammatory diseases, and a composition for diagnosing inflammatory diseases are disclosed.