Digital signatures are important applications of public key cryptography in today’s digital networks. However, they have a problem that anyone can verify the signature even if a signer wants to restrict the verifiers to confirm his signatures. D. Chaum et al.[1] proposed undeniable signatures to solve this problem. These signatures are based on the discrete logarithm problem and are extended to those with different properties [2]-[4]. After that, R.Gennaro et al.[5] proposed another undeniable signature scheme based on RSA. However, this scheme also has following problems. Firstly, the undeniable signature of them cannot be converted into a usual signature individually. So if a user wants to use both of the undeniable signature and the usual signature, he must prepare separate parameters for each type of signatures. Secondly, the denial protocol is not deterministic because it uses a zero knowledge interactive proof. So it is not efficient. Thirdly, their signature system cannot resist hidden verifier attack[7]. In this paper we will propose an improved scheme to solve these problems.