the control arm as shown in fig3 both left and right sides which form left and right cavities 63 64 from a main cavity 62 having a closed space therein each have a closed space of the same width such that closed spaces having double sides are formed at both sides inside body plate 6 the width b of main cavity 62 is the same as the width of bush fastening end 9 formed at an end of the control arm and the width a between left and right side cavities 63 64 including main cavity 62 is the same as the width of carrier fastening end 8 formed at the opposite side to bush fastening end 9 of the control arm in addition both outer sides of body plate 6 is inclined by forming slopes 66 at lower end of the outer sides of left and right cavities 63 64 that form both sides of main cavity 62 further in the thickness of the control arm the entire thickness of body plate 6 is set to 2 5 mm to reduce the weight of the product while main cavity 62 and left and right cavities 63 64 that making the closed hollow structure with double walls have various thicknesses for example a main reference thickness a 1 of the wall separating main cavity 62 from left and right cavities 63 64 is set to about 3 0 mm whereas the side reference thickness b 1 forming the outer walls of left and right cavity 63 64 is set to about 4 0 mm further a rounded connecting portion.