The hydromagnetic plane steady flow of an infinitely conducting gas over a thin infinite wave-shaped wall is discussed under the condition that the directions of the flow velocity and magnetic field are in the same plane. When the external magnetic field is parallel to the undisturbed stream, the flow is shown to be current free and irrotational except for some surface currents. For the case of extennal magnetic field perpendicular to the undisturbed stream, the hydromagnetic effects consist of the waves of electric currents and vorticity exterding from the body. The drag was determinsd both for subsonic ard supersonic cases.