An open cable set-top box diagnosing system in which a point of deployment (POD), separated from a main circuit unit, having a newly defined interface is used to check an operation state of an open cable set-top box, transmits the checked state to a predetermined server, according to which the operation state of the open cable set-top box is diagnosed. The open cable set-top box diagnosing system includes an open cable set-top box for checking its own operation state by using a diagnosis resource by a communication protocol between a point of deployment (POD) separated from the main circuit unit and the main circuit, and a head end for providing a service corresponding to a request signal received from the open cable set-top box or providing a broadcast program to the open cable set-top box, and checking the operation state of the open cable set-top box.