A multi-link piston crank mechanism for an internal combustion engine, including a crankshaft having a crank pin, and crank webs connected with each other through the crank pin. A pair of thrust surfaces are formed on the crank webs so as to be opposed in a direction of a center axis of the crank pin. A first link pivotally supported on the crank pin is coupled to a second link through a connection pin located in a space between the thrust surfaces. The connection pin is moveable about the crank pin to form a circular trace along which the thrust surfaces extend. The end surfaces of the connection pin and the thrust surfaces come into mutual contact to prevent the connection pin from falling out of the space between the thrust surfaces.