“Diploma” is issued only when you graduate or finish your course, so we don’t recommend you to use diploma for attestation. Once attested, you cannot use it anymore. Highly recommend you to get “Graduation Certificate” 卒業証明書 (Sotsugyo-shomeisho) in Japanese.
Depending on the University, you can directly apply for the issuance online, but some University needs Power of Attorney and third agent in Japan. Therefore, please contact your University first and check the process.
*University in Japan, they don’t certify the true copy of the diploma