A lower link of a double link type piston crank mechanism comprises a crank pin bearing housing portion that is adapted to receive a crank pin of a crankshaft and defines a first contour, an upper pin receiving bore portion that is adapted to receive an upper pin of an upper link and defines a second contour, a control pin receiving bore portion that is adapted to receive a control pin of a control link and defines a third contour, and a given portion that has therein an internally threaded bore formed in one of upper and lower half-parts of the lower link and a bored portion formed in the other of the upper and lower half-parts of the lower link, the given portion defining a fourth contour. The upper and lower half-parts are coupled by a bolt that passes through the bored portion and is engaged with the internally threaded bore. The lower link has further radially projected portions that extend radially outward beyond an imaginary minimum reference contour that is provided by connecting outer edge portions of the first, second, third and fourth contours with a continuous line.