To provide a highly versatile security system for reducing installation cost, without requiring large-scale work.

This security system has a key control device arranged in an existing key arranged in a window and a door and controlling locking and unlocking of the existing key, a lock device arranged in a predetermined position of the window and the door and locking and unlocking the window and the door separately from the existing key, a security management device performing radiocommunication with the key control device and the lock device, and a portable terminal performing radiocommunication with the security management device. The portable terminal has application software for controlling the key control device, the lock device and the security management device. The portable terminal controls the key control device and the lock device via the security management device by performing radiocommunication after performing predetermined operation by starting the application software.