The purpose of this study is to find a basis of binding force for automated smart contracts. In order to achieve that purpose, the author attempts a new exploration of the traditional contract theory. The modern society is digitized and is changing into the era of smart contracts. Accordingly, the formation and performance of traditional contracts according to the agreement of intentions are also changing. Due to the automation of contracts, human interaction is being excluded from existing contracts. How to form the basis for the agreement of the parties' will, its binding force and responsibility? In smart contracts, reliability guarantee by consensus protocol is important. Accordingly, the discussion at the level of contract theory also needs a change. So, it was discussed as follows. First of all, the core contents of will theory, promise theory and reliance theory as conventional contract theories were examined. And in the significance of smart contracts, the characteristics of contract requirements were found. This was applied to each contract theory and the appropriateness was reviewed. Based on the above, protection of reliance was emphasized as a basis of binding force for smart contracts. A contract theory that protects the user's reliance and considers the intention of the parties was proposed. In smart contract relationships with characteristics such as automation and anonymity, first of all, a reliance-based theory was approached to help protect user reliance. It is to consider the will or agreement for forming such reliance. In this case, the concept of reliance is a reasonable and legitimate general trust.