A double-link piston crank mechanism for an internal combustion engine comprising: a lower link rotatably installed on a crankpin of a crankshaft; an upper link rotatably connected at one end to a piston pin of a piston and rotatably connected at the other end configured as a pin boss part to the lower link through a first connecting pin; a control link supported at one end by an engine body side and rotatably connected at the other end configured as a pin boss part to the lower link; a crankpin oil passage formed to radially extend inside of the crankpin, one end of the crankpin oil passage being structured to open to an outer circumferential surface of the crankpin; a pin-boss oil passage formed in at least one of the pin boss part of the upper link and the pin boss part of the control link, one end of the pin-boss oil passage being structured to open to an inner circumferential surface of the at least one pin boss part (14; 19) and the other end of the pin-boss oil passage being structured to open to an outer circumferential surface of the at least one pin boss part (14; 19); a lower-link oil passage formed in the lower link, one end of the lower-link oil passage structured to open to a pin-boss-facing surface (33; 34) facing the outer circumferential surface of the at least one pin boss part (14; 19) and the other end of the lower-link oil passage structured to open to a crankpin bearing surface; and the lower-link oil passage configured to be brought into fluid-communication with the crankpin oil passage at a prescribed crank angle for jetting out lubricating oil from an opening on the one end side of the lower-link oil passage, characterised in that the one-end-side opening of the lower-link oil passage is formed so as to point, at the prescribed crank angle, to a specified end edge of end edges of an opening on the other end side of the pin-boss oil passage as viewed in an axial direction of the crankshaft, the specified end edge facing on a traveling direction side of the at least one pin boss part (14; 19) in which the pin-boss oil passage is formed. a lower link rotatably installed on a crankpin of a crankshaft; an upper link rotatably connected at one end to a piston pin of a piston and rotatably connected at the other end configured as a pin boss part to the lower link through a first connecting pin; a control link supported at one end by an engine body side and rotatably connected at the other end configured as a pin boss part to the lower link; a crankpin oil passage formed to radially extend inside of the crankpin, one end of the crankpin oil passage being structured to open to an outer circumferential surface of the crankpin; a pin-boss oil passage formed in at least one of the pin boss part of the upper link and the pin boss part of the control link, one end of the pin-boss oil passage being structured to open to an inner circumferential surface of the at least one pin boss part (14; 19) and the other end of the pin-boss oil passage being structured to open to an outer circumferential surface of the at least one pin boss part (14; 19); a lower-link oil passage formed in the lower link, one end of the lower-link oil passage structured to open to a pin-boss-facing surface (33; 34) facing the outer circumferential surface of the at least one pin boss part (14; 19) and the other end of the lower-link oil passage structured to open to a crankpin bearing surface; and the lower-link oil passage configured to be brought into fluid-communication with the crankpin oil passage at a prescribed crank angle for jetting out lubricating oil from an opening on the one end side of the lower-link oil passage, characterised in that the one-end-side opening of the lower-link oil passage is formed so as to point, at the prescribed crank angle, to a specified end edge of end edges of an opening on the other end side of the pin-boss oil passage as viewed in an axial direction of the crankshaft, the specified end edge facing on a traveling direction side of the at least one pin boss part (14; 19) in which the pin-boss oil passage is formed.