An interactive digital signage system which allow users to fetch detailed information using their smartphone is widely deployed. Bluetooth Low Energy(BLE) broadcast beacon is a one of possible method to broadcast a detailed information to users. However, BLE beacon doesn't provide a way to assay an authenticity of beacon since BLE broadcast is insecure untrusted communication. Out-of-band authentication is used to improve trustworthiness for such untrusted pathway. A new authentication protocol is proposed to assay the authenticity of digital signage when the information is broadcasted via BLE beacon. The authors implement the protocol and evaluate its effectiveness.