Abstract: A trading system for atomically executing a smart contract for trading between a first blockchain and a second blockchain includes a coordinator section and a first user section on the first blockchain and a second user section on the second blockchain. The coordinator section sends a first packet created based on a predetermined transaction in which a contract function is specified to the first and second user sections. When the contract function in the first packet received from the coordinator section is successful, the first user section saves a change operation in a state store associated therewith, acquires lock for a change target, and sends a second packet with a status set executable to the coordinator section. When the execution is failed, the first user section discards the change operation in the state store associated therewith and sends a third packet with the status set inexecutable to the coordinator section.