PKI(Public Key Infrastructure) is used to enforce legal binding forces to the Internet services. The PKI certificate is the legal certification of the user's public key and is issued by the trustworthy CA(Certificate Authority) designated by the government. Therefore, users can create legally binding signatures based on PKI certificates. Because biometric data is unique to each person, biometric authentication can legally prove one's identity without the help of PKI system. As users participating in Internet services such as social media are not subject to regional restrictions, thus it is not easy to implement Internet services based on PKI. Blockchain is a technology that stores data that cannot be modified or deleted, and does not require the help of a trusted third party. The block to add is determined by consensus of users participating in the blockchain network. In this study, we propose the undeniable biometric signature scheme based on Ethereum smart contracts. The proposed scheme consists of biometric template registration, undeniable signature generation, and verification protocolss. The Undeniable signature cannot be verified without the help of the signer. The signer uses a challenge-response protocol to show the legitimacy of the signature only to the desired verifier. The proposed scheme can be extended to a multi-signature scheme by modifying the El-Gamal signature scheme and can increase the reliability of blockchain-based Internet services.