This paper studies security against truncated differential cryptanalysis from the “designer’s” standpoint. In estimating the security, we use the upper bound of truncated differential probability. Previous works, Knudsen, Matsui and Moriai et al., searched for effective truncated differentials to attack byte-oriented block ciphers and computed the exact probability of the differentials. In this paper, we discuss the following items from the designer’s standpoint; (a) truncated differential probability of effective active-s-box, (b) XOR cancellation probability, and (c) effect of auxiliary functions, e.g., FL/FL ^-1-functions. We then combine them with Matsui’s search algorithm and evaluate the security of Camellia, jointly developed by NTT and Mitsubishi Electric Corporation, against truncated differential cryptanalysis. We prove (from the designer’s standpoint) that variants of Camellia with more than 11 rounds are secure against truncated differential cryptanalysis even if weak-key FL/FL ^-1-functions are taken into consideration.