A method of forming a stress relieving portion (S; S1; S2) for relieving stress on a circuit board (40; 140; 240; 340; 440; 540) fastened to a predetermined part by a fastener (44a; 44b; 44c; 44d; 44e; 44f; 44g), the circuit board (40; 140; 240; 340; 440; 540) having a plurality of fastening holes (42a; 42b; 42c; 42d; 42e; 42f; 42g) used to fasten the circuit board (40; 140; 240; 340; 440; 540) to the predetermined part, the fastening holes (42a; 42b; 42c; 42d; 42e; 42f; 42g) including a first fastening hole and a second fastening hole, the circuit board (40; 140; 240; 340; 440; 540) comprising a plurality of electronic components (Cap; Coil; Cn; IC; M; R) mounted on a surface of the circuit board (40; 140; 240; 340; 440; 540), wherein the method comprises: determining a predetermined distance which is a threshold value of a distance between two fastening holes at which the rigidity of the circuit board against vibration that is input into the circuit board having been fastened to the predetermined part by the fasteners, starts to decrease due to formation of the stress relieving portion, measuring a distance between the first fastening hole and the adjacent second fastening hole, and forming the stress relieving portion around the first fastening hole in a region that does not intersect with a straight line (Lref) connecting the first fastening hole and the second fastening hole located adjacent to the first fastening hole when the distance between the first fastening hole and the second fastening hole is equal to or larger than the predetermined distance and an electrolytic capacitor (Cap) or a coil (Coil) or a transistor or no electronic component is disposed on the straight line, forming the stress relieving portion around the first fastening hole in a region that does intersect with a straight line (Lref) connecting the first fastening hole and the second fastening hole located adjacent to the first fastening hole when the distance between the first fastening hole and the second fastening hole is larger than or equal to the predetermined distance (Dref) and a resistor element or an integrated circuit is disposed on the straight line (Lref), and forming the stress relieving portion in a region intersecting with the straight line (Lref) connecting the first fastening hole and the adjacent second fastening hole when the distance (Dh) is smaller than the predetermined distance (Dref). determining a predetermined distance which is a threshold value of a distance between two fastening holes at which the rigidity of the circuit board against vibration that is input into the circuit board having been fastened to the predetermined part by the fasteners, starts to decrease due to formation of the stress relieving portion, measuring a distance between the first fastening hole and the adjacent second fastening hole, and forming the stress relieving portion around the first fastening hole in a region that does not intersect with a straight line (Lref) connecting the first fastening hole and the second fastening hole located adjacent to the first fastening hole when the distance between the first fastening hole and the second fastening hole is equal to or larger than the predetermined distance and an electrolytic capacitor (Cap) or a coil (Coil) or a transistor or no electronic component is disposed on the straight line, forming the stress relieving portion around the first fastening hole in a region that does intersect with a straight line (Lref) connecting the first fastening hole and the second fastening hole located adjacent to the first fastening hole when the distance between the first fastening hole and the second fastening hole is larger than or equal to the predetermined distance (Dref) and a resistor element or an integrated circuit is disposed on the straight line (Lref), and forming the stress relieving portion in a region intersecting with the straight line (Lref) connecting the first fastening hole and the adjacent second fastening hole when the distance (Dh) is smaller than the predetermined distance (Dref).