Phenylalanine derivatives of the following formula, analogues thereof and pharmaceutically acceptable salts thereof have an antagonistic activity to α4 integrin. They are used as therapeutic agents or preventive agents for various diseases concerning α4 integrin in which α4 integrin-depending adhesion process participates in the pathology, such as inflammatory diseases, rheumatoid arthritis, inflammatory bowel diseases, systemic lupus erythematosus, multiple sclerosis, Sjögren's syndrome, asthma, psoriasis, allergy, diabetes, cardiovascular diseases, arterial sclerosis, restenosis, tumor proliferation, tumor metastasis and transplantation rejection. wherein R1 is a methyl group and R2 is a methoxy group.