solid line 43 with such convex shaping of mating surface 41 two spaced walls 42 are deformed inward toward each other as is exaggeratedly shown by solid lines 44 however in the present invention due to provision of the projected portion 33 the undesired inward deformation of two spaced walls 42 is suppressed or at least minimized in other words the portion of lower link 1 that surrounds the apex 34 serves as a reinforcing means of the annular boss portion 9 of lower link 1 as is seen from fig1 pin bore 10 for the crank pin is formed in an area of lower link 1 where a center of gravity is provided this is advantageous for reducing undesired vibration of lower link 1 relative to the crank pin as is seen from fig1 the projected portion 23 and the other projected portion 33 are arranged at generally opposite positions with respect to the center 10 c of pin bore 10 for the crank pin this arrangement promotes reduction of the undesired vibration of lower link 1 as is seen from fig1 lower link 1 is integrally formed at an outer side of annular boss portion 7 for the upper pin with a.