A belt transmission mechanism has a crowned drive pulley, a crowned driven pulley, and a belt trained under tension around the crowned drive pulley and the crowned driven pulley. The crowned drive pulley and the crowned driven pulley are dimensioned to satisfy the equation:D2/D1=k.multidot.R1/R2where D1 is the diameter of the crowned drive pulley, D2 is the diameter of the crowned driven pulley, R1 is the radius of the crown of the crowned drive pulley, R2 is the radius of the crown of the crowned driven pulley, and k is a constant which is approximately 1. The belt transmission mechanism with the above drive and driven pulley dimensions can maintain its speed reduction ratio highly accurately even when the tension of the belt varies.