A blockchain has attracted in many research fields. In this paper, a blockchain-based distributed optimization method for energy management systems (EMSs) is studied. In distributed optimization, the original problem is decomposed, and the local problems are solved by multiple agents. Hence, distributed optimization is appropriate for large-scale multi-agent systems. In this paper, ADMM (Alternating Direction Method of Multipliers) is utilized as one of the powerful methods in distributed optimization. In ADMM, an aggregator is required for collecting the computation result in each agent. Using blockchains, the function of an aggregator can be contained in a distributed ledger, and an aggregator may not be required. As a result, tampering from attackers can be prevented. In this paper, after ADMM and blockchains are summarized, an EMS studied in this paper is introduced. By numerical experiments, the effectiveness and limitation of blockchain-based distributed optimization are clarified from the viewpoints of tamper tolerance and computation time.