A method and a device for monitoring an elevator car door acquire one or more distances based on light emitted toward the car door by a distance measuring device and reflected at the car door as a function of a current closed state of the car door. Information about the current closed state of the car door is derived solely by comparing the acquired distance with a previously acquired reference distance, and a signal which represents the information can be fed to a remotely arranged control center. The monitoring device can be installed as a retrofit component that is capable of working autonomously in existing elevator systems, does not need to receive any data from a controller of the car door or the elevator system and only requires a relatively simple data processing device that only needs to carry out a comparison of detected distance values with reference values.