A core assembly comprising first and second core members each having a rectangular body around which an X-axis coil and a Y-axis coil are wound, and flanges integrally and diagonally extending from the body; and a bobbin having an annular portion and projections diagonally extending therefrom; the projections of the bobbin being provided with terminal members connected to coil ends of the X-axis coil, the Y-axis coil and the Z-axis; the annular portion of the bobbin acting as a space for disposing the first core member from one side, and providing a space receiving at least partially the body of the second core member from the other side, such that the body of the first core member is at least partially adjacent to the body of the second core member; and a space for winding the Z-axis coil being provided between the projections of the bobbin and the flanges of the second core member.