A stepless speed-change power transmission adapted to be connected with a vehicle engine, the vehicle engine including a crankshaft box formed with an intake port at a front end thereof, and an input shaft extending from the crankshaft box, said stepless speed-change power transmission including: a transmission case adapted to be connected with the crankshaft box and formed with an opening in fluid communication with said intake port in said transmission case, said input shaft extending into said transmission case through said opening; a stepless speed-change unit including a driving pulley disposed on said input shaft and disposed in said transmission case, an output shaft disposed in said transmission case and adapted to be disposed behind said input shaft, a driven pulley disposed on said output shaft and in said transmission case, and a transmission belt trained on said driving pulley and said driven pulley, said driving pulley including a fixed driving pulley half and a movable driving pulley half, said driven pulley including a fixed driven pulley half and a movable driven pulley half, said transmission belt being disposed between said fixed driving pulley half and said movable driving pulley half, and between said fixed driven pulley half and said movable driven pulley half; and an air guide unit for guiding air to flow in said transmission case; characterized by: said air guide unit including a curved first air guide wall disposed in said transmission case and around said fixed driving pulley half, and a curved second air guide wall disposed in said transmission case and around said fixed driving pulley half and spaced apart from said first air guide wall to form first and second notches (237, 238) between said first and second air guide walls (231, 234), said first notch being positioned so as to guide air from said fixed driving pulley half onto said driven pulley, said second notch being positioned so as to guide from said fixed driving pulley half onto said movable driving pulley half. a transmission case adapted to be connected with the crankshaft box and formed with an opening in fluid communication with said intake port in said transmission case, said input shaft extending into said transmission case through said opening; a stepless speed-change unit including a driving pulley disposed on said input shaft and disposed in said transmission case, an output shaft disposed in said transmission case and adapted to be disposed behind said input shaft, a driven pulley disposed on said output shaft and in said transmission case, and a transmission belt trained on said driving pulley and said driven pulley, said driving pulley including a fixed driving pulley half and a movable driving pulley half, said driven pulley including a fixed driven pulley half and a movable driven pulley half, said transmission belt being disposed between said fixed driving pulley half and said movable driving pulley half, and between said fixed driven pulley half and said movable driven pulley half; and an air guide unit for guiding air to flow in said transmission case; characterized by: said air guide unit including a curved first air guide wall disposed in said transmission case and around said fixed driving pulley half, and a curved second air guide wall disposed in said transmission case and around said fixed driving pulley half and spaced apart from said first air guide wall to form first and second notches (237, 238) between said first and second air guide walls (231, 234), said first notch being positioned so as to guide air from said fixed driving pulley half onto said driven pulley, said second notch being positioned so as to guide from said fixed driving pulley half onto said movable driving pulley half.