A lower link (13) is formed such that outer circumferential sides of both end portions of a crankpin through-hole (21) are stiffer than a center portion of the crankpin through-hole (21), i.e. a bifurcation portion of a bifurcated shape thereof. The both end portions of the crankpin through-hole (21) are formed such that inner circumferential surfaces (22a) of the both end portions are curved at a predetermined curvature in an axial direction of a crankshaft with no load input on the lower link (13). Moreover, the center portion of the crankpin through-hole (21) is formed such that an inner circumferential surface (22b) thereof is located inward of the inner circumferential surfaces (22a) of the both end portions and is straight in the axial direction of the crankshaft with no load input on the lower link (13).