In an internal combustion engine, comprising: an upper link (11) connected via a piston pin (21) to a piston (32) that reciprocates within a cylinder; a lower link (12) attached to a crank pin (33b) of a crankshaft (33) to be free to rotate and connected to the upper link (11) via an upper pin (22); and a control link (13) which is connected to the lower link (12) via a control pin (23) and oscillates about an oscillation central shaft (24), the following equation is established when the piston (32) is at bottom dead center cos(θl+α)