A monitor device for use with an endoscope apparatus, which has first and second display modes. In the first display mode, the monitor device displays, in real time, any image transmitted in a normal radio communication state. In the second display mode, the monitor device displays a noise-free image in real time, if a radio communication error occurs. If one-frame (or one-field) image data is not received within the frame (or field) time due to a communication error, the monitor device displays, in the second display mode, the image data received in a normal state immediately before the one-frame (or one-field) image data, and informs the operator of the communication error.