To provide a circuit board capable of reducing high-frequency noise generated from an electronic circuit mounted to the circuit board.

A main board 10 is equipped with an electronic circuit having the source of high-frequency noise. The main board 10 includes a conductor which is formed near the electronic circuit and has a loop-shaped closed path not electrically connected to the electronic circuit. The conductor having the loop-shaped closed path causes electromagnetic induction in response to high-frequency noise to generate a magnetic field. The magnetic field generated from the conductor and a magnetic field generated from the electronic circuit then are canceled out from each other, thus reducing high-frequency noise at the end.