A method and a device for detecting the opening state of a garage door or the like, such as a door that restricts entry by a vehicle are provided. A control apparatus generates a predetermined light pattern. A headlight apparatus projects the predetermined light pattern onto the door, while a camera apparatus detects the light pattern projected onto the door. The control apparatus compares the light pattern detected by the camera apparatus with the predetermined light pattern and outputs information signals according to the differences between the detected light pattern and the predetermined light pattern. The differences determine the opening state of the door.