A coupler includes a first conductive pattern provided on a substrate having insulating property, a second conductive pattern provided on the substrate and placed in opposition to the first conductive pattern, a third conductive pattern provided on the substrate, a fourth conductive pattern provided on the substrate and placed in opposition to the third conductive pattern, a ground potential portion placed around positions on the substrate where the first conductive pattern, the second conductive pattern, the third conductive pattern, and the fourth conductive pattern are placed, a first resistor connecting between the first conductive pattern and the second conductive pattern placed in opposition to each other, and a second resistor connecting between the third conductive pattern and the fourth conductive pattern placed in opposition to each other.