sentences: IoT (Internet of Things) has a large portion of our life. This is manifested by the large number of connected devices. With the exponential growth of IoT devices, IoT security is becoming important. In particular, Smart Door Lock system is extremely important because it is closely related to the safety of the user. However, the data sent and received of existing Smart Door Lock system is vulnerable to forgery and hacking. To improve these security issues, we propose a Smart Door Lock system based on blockchain. Also, this provides data integrity and non-repudiation. Lastly, we propose an algorithm that the Smart Door Lock system judge some situations around itself and operates based on data sent from sensors.