To set the resolution with the simple operation while enabling the connection of a personal computer and a display device of different makers to each other.

This adapter device is provided with a sensor signal generating circuit 10 for generating the discriminating information for setting the resolution inside of the adapter device for connecting a personal computer device and a display device to each other, and switch terminals 5a, 5b for switching a first and a second connecting mode of the sensor signal generating circuit 10 can be operated for ON/OFF by one operating element. The personal computer device sets the resolution on the basis of the discriminating code obtained by the connecting mode of the sensor signal generating circuit 10 and outputs the image signal for image display.