A lower link of a piston crank mechanism of an internal combustion engine, wherein the lower link is of a split type including upper and lower half-parts (2, 3) adapted to be coupled by a bolt (4, 5), and wherein the lower link is adapted to be pivotally connected to one end of an upper link through an upper pin, is adapted to be pivotally disposed through a crank pin bore on a crank pin of a crankshaft and is adapted to be pivotally connected to one end of a control link through a control pin, and wherein a contacting surface of the two half-parts passes through a center of the crank pin bore, the lower link comprising: a crank pin bearing housing portion adapted to receive the crank pin, the crank pin bearing housing portion being an annular portion which defines a first contour in a side view of the lower link, an upper pin receiving bore portion adapted to receive the upper pin, the upper pin receiving bore portion being an annular boss portion which defines a second annular contour in the side view of the lower link, and a control pin receiving bore portion adapted to receive the control pin, the control pin receiving bore portion being an annular boss portion which defines a third annular contour in the side view of the lower link, characterized by a given portion (16, 18; 17, 19) that has therein an internally threaded bore (12, 13) formed in one of the upper and lower half-parts (2, 3) of the lower link and a bored portion (14, 15) formed in the other of the upper and lower half-parts (2, 3) of the lower link, the given portion (16, 18; 17, 19) being a bulged portion thicker than a general portion of the lower link and defining a fourth rectangular contour in the side view of the lower link, the upper and lower half-parts (2, 3) being coupled by the bolt (4, 5) that passes through the bored portion (14, 15) and is engaged with the internally threaded bore (12, 13); and radially projected portions (23, 33) 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, wherein an apex of one of the radially projected portions is positioned at the side of the upper pin receiving bore portion with respect to a normal bisector of a straight line segment that connects a center of the upper pin receiving bore portion and the center of the crank pin bearing housing portion, and wherein, an apex of the other one of the radially projected portions is positioned at the side of the control pin receiving bore portion with respect to a normal bisector of a straight line segment that connects a center of the control pin receiving bore portion and the center of the crank pin bearing housing portion, and wherein the upper half-part includes the upper pin receiving bore portion and the lower half-part includes the control pin receiving bore portion. a crank pin bearing housing portion adapted to receive the crank pin, the crank pin bearing housing portion being an annular portion which defines a first contour in a side view of the lower link, an upper pin receiving bore portion adapted to receive the upper pin, the upper pin receiving bore portion being an annular boss portion which defines a second annular contour in the side view of the lower link, and a control pin receiving bore portion adapted to receive the control pin, the control pin receiving bore portion being an annular boss portion which defines a third annular contour in the side view of the lower link, a given portion (16, 18; 17, 19) that has therein an internally threaded bore (12, 13) formed in one of the upper and lower half-parts (2, 3) of the lower link and a bored portion (14, 15) formed in the other of the upper and lower half-parts (2, 3) of the lower link, the given portion (16, 18; 17, 19) being a bulged portion thicker than a general portion of the lower link and defining a fourth rectangular contour in the side view of the lower link, the upper and lower half-parts (2, 3) being coupled by the bolt (4, 5) that passes through the bored portion (14, 15) and is engaged with the internally threaded bore (12, 13); and