To provide a safe and stable medicine useful for prevention and treatment of prostate cancer.A novel PI polyamide is acquired that recognizes and binds to a specific base sequence of an Oct1 gene binding sequence present in a transcriptional regulatory region (AR response region) of an ACSL3 gene and regulating the transcription activity of AR. This leads to the provision of an ACSL3 gene expression inhibitor and a preventive and/or therapeutic agent of prostate cancer containing the PI polyamide as an active ingredient.