Positions of a plurality of objects in a space are detected. A position of reflected light of an object passing through an infrared screen is specified by analyzing an image obtained by selectively imaging the infrared rays. When the infrared screen is formed in front the display, reflected light is only caused just in front of the display. When the infrared rays are selectively imaged, a picture displayed with visible light is separated from reflected light in the infrared region, and only the reflected light can be imaged. A position of the reflected light on the display can be specified by a publicly known image analysis technique.