A monitor device is provided with an imaging device that shoots an overhead line and a current collector, and a controller that processes an image. The controller includes a day or night determination processing section, and an image processing section that switches a parameter for recognizing the overhead line and the current collector in the image by executing image processing different in the daylight and at night based upon the result of the day or night determination. Further, there are provided reference photographic subjects to be shot by the imaging device in positions different from the overhead line and the current collector in an image area to be shot, and the day or night determination processing section performs the determination of day or night based upon a luminance average value of the reference photographic subjects inputted into an image input section.