NTRUSign is a lattice-based digital signature scheme proposed by Hoffstein et al. NTRUSign is quite different from many other signature schemes in a sense that its security depends on neither the integer factorization problem nor the discrete logarithm problem but on a geometric problem called the close vector searching problem. However, it is known that there is some vulnerability in NTRUSign, namely there is an attack called the transcript attack. In this paper, we propose a countermeasure for protecting NTRUSign against the transcript attack, and give an improved NTRUSign algorithm.