We investigate the complexity of breaking cryptosystems of which security is based on the discrete logarithm problem. We denote the algorithms of breaking the Diffie-Hellman’s key exchange scheme by DH, the Bellare-Micali’s non-interactive oblivious transfer scheme by BH, the ElGamal’s public-key cryptosystem by EG, the Okamoto’s conference-key sharing scheme by CONF, and the Shamir’s 3-pass key-transmission scheme by BPASS, respectively. We show a relation among these cryptosystems that (Formula Presented) where (Formula Presented) denotes the polynomial-time functionally many-teone reducibility, i.e. a function version of the (Formula Presented) -reducibility. We further give some condition in which these algorithms have equivalent difficulty. Namely, 1. If the complete factorization of p - 1 is given, i.e. if the the discrete logarithm problem is a certified one, then these cryptosystems are equivalent w.r.t. expected polynomial-time functionally Turing reducibility. 2. If the underlying group is the Jacobian of an elliptic curve over 2pwith a prime order, then these cryptosystems are equivalent w.r.t. polynomial-time functionally many-to-one reducibility. We also discuss the complexity of several languages related to those computing problems.