A control device of an elevator, comprising: a car door provided in a car of the elevator; a door driving device which is provided in the car and drives the car door; a hall door provided in a hall of the elevator; a connecting device which causes the hall door to open and close in synchronization with the car door when the car has stopped at the hall; a car door switch which is provided in the car to detect that the car door is in a prescribed fully closed condition and goes into action when the car door has reached a prescribed position; run control means which causes the car to stop at the hall when prescribed requirements for conducting a failure diagnosis of the car door switch are met; door opening and closing control means which causes the car door to open beyond an action position of the car door switch after the car is stopped at the hall by the run control means and causes the car door to close without causing the car door to open to a width which is not less than a prescribed specified width through which an elevator user cannot pass; and failure determination means which makes a determination as to the existence or nonexistence of a failure of the car door switch on the basis of the action condition of the car door switch occurring when the car door is opened and closed by the door opening and closing control means, wherein the connecting device has such a prescribed structure that causes the door opening action of the hall door from the fully closed condition to be started later than the door opening action of the car door; and the door opening and closing control means causes the car door to open and close without causing the hall door to perform opening and closing actions after the car is stopped at the hall by the run control means. a car door provided in a car of the elevator; a door driving device which is provided in the car and drives the car door; a hall door provided in a hall of the elevator; a connecting device which causes the hall door to open and close in synchronization with the car door when the car has stopped at the hall; a car door switch which is provided in the car to detect that the car door is in a prescribed fully closed condition and goes into action when the car door has reached a prescribed position; run control means which causes the car to stop at the hall when prescribed requirements for conducting a failure diagnosis of the car door switch are met; door opening and closing control means which causes the car door to open beyond an action position of the car door switch after the car is stopped at the hall by the run control means and causes the car door to close without causing the car door to open to a width which is not less than a prescribed specified width through which an elevator user cannot pass; and failure determination means which makes a determination as to the existence or nonexistence of a failure of the car door switch on the basis of the action condition of the car door switch occurring when the car door is opened and closed by the door opening and closing control means, wherein the connecting device has such a prescribed structure that causes the door opening action of the hall door from the fully closed condition to be started later than the door opening action of the car door; and the door opening and closing control means causes the car door to open and close without causing the hall door to perform opening and closing actions after the car is stopped at the hall by the run control means.