A power transmission system comprising: an input shaft; an output shaft disposed coaxially with the input shaft; a supercharger casing configured to support the input shaft and the output shaft; a transmission mechanism connected between the input shaft and the output shaft; and an injection passage to supply a portion of oil for lubricating one of the input shaft and the output shaft, to the other of the input shaft and the output shaft by means of a jetting spray that traverses an interior space of the transmission mechanism, which oil lubricates bearings that support the one of the shafts (44, 65), wherein: the transmission mechanism is in the form of a planetary gear transmission mechanism; the one of the shafts (44, 65) is the output shaft having a sun gear of the planetary gear transmission mechanism, which sun gear is provided on a transmission side axial end portion of the output shaft; wherein the input shaft is rotatably supported by an input shaft casing portion of the supercharger casing, and the output shaft is rotatably supported by an output shaft casing portion of the supercharger casing via the bearings, wherein the injection passage is disposed on a radially outward side of the one of the shafts (44, 65); and characterized in that via the injection passage, a portion of the oil for lubricating the bearings supporting the output shaft is jetted from the output shaft side towards the input shaft in between the sun gear and a ring gear of the planetary gear transmission mechanism the input shaft casing portion and the output shaft casing portion being connected with each other with the use of bolts; the output shaft casing portion is formed with the injection passage branched off from a lubricant oil passage; an axis of the injection passage passes on the radially outward side of the one of the shafts (44, 65). an input shaft; an output shaft disposed coaxially with the input shaft; a supercharger casing configured to support the input shaft and the output shaft; a transmission mechanism connected between the input shaft and the output shaft; and an injection passage to supply a portion of oil for lubricating one of the input shaft and the output shaft, to the other of the input shaft and the output shaft by means of a jetting spray that traverses an interior space of the transmission mechanism, which oil lubricates bearings that support the one of the shafts (44, 65), wherein: the transmission mechanism is in the form of a planetary gear transmission mechanism; the one of the shafts (44, 65) is the output shaft having a sun gear of the planetary gear transmission mechanism, which sun gear is provided on a transmission side axial end portion of the output shaft; wherein the input shaft is rotatably supported by an input shaft casing portion of the supercharger casing, and the output shaft is rotatably supported by an output shaft casing portion of the supercharger casing via the bearings, wherein the injection passage is disposed on a radially outward side of the one of the shafts (44, 65); and characterized in that via the injection passage, a portion of the oil for lubricating the bearings supporting the output shaft is jetted from the output shaft side towards the input shaft in between the sun gear and a ring gear of the planetary gear transmission mechanism the input shaft casing portion and the output shaft casing portion being connected with each other with the use of bolts; the output shaft casing portion is formed with the injection passage branched off from a lubricant oil passage; an axis of the injection passage passes on the radially outward side of the one of the shafts (44, 65). the transmission mechanism is in the form of a planetary gear transmission mechanism; the one of the shafts (44, 65) is the output shaft having a sun gear of the planetary gear transmission mechanism, which sun gear is provided on a transmission side axial end portion of the output shaft; wherein the input shaft is rotatably supported by an input shaft casing portion of the supercharger casing, and the output shaft is rotatably supported by an output shaft casing portion of the supercharger casing via the bearings, wherein the injection passage is disposed on a radially outward side of the one of the shafts (44, 65); and characterized in that via the injection passage, a portion of the oil for lubricating the bearings supporting the output shaft is jetted from the output shaft side towards the input shaft in between the sun gear and a ring gear of the planetary gear transmission mechanism the input shaft casing portion and the output shaft casing portion being connected with each other with the use of bolts; the output shaft casing portion is formed with the injection passage branched off from a lubricant oil passage; an axis of the injection passage passes on the radially outward side of the one of the shafts (44, 65).