The present invention provides a service method of rental storage, which, when a rental storage service provider provides a rental storage for rental storage service users, allows ideal use of storages in correspondence with the billing charge to the users by proposing the most optimum contract options to the users, and which allows suppressing the management cost of the users. The service method of rental storage through network connection, the storage provider will estimate the future usage of storage based on the history of usage of storage by the rental storage service user to report the estimation to the user. The rental storage service user will update the contract by expanding or reducing the amount of data usage so as to receive the most optimum contract conditions of rental storage service.