To provide a friction transmission device formed in a simple structure, simplifying the processing and shape inspection, and in which the ratio of a pressing force to a transmission force can be more increased as the transmission force from one roller to the other is increased while preventing an excessive pressing force from occurring.

This friction transmission device comprises a cam face 51 having an angle relative to a tangent at a contact point between a drive roller 1 and a driven roller 2. When the center of a drive roller 1 under no-load is O1, the center of a driven roller 2 is O2, an xy coordinate is set with O1 taken as an origin, a y-axis is passed through O2, and a distance between O1 and O2 is r0, the cam face 51 abuts on the rotating support part of the drive roller 1 to pressingly contact the rollers on each other and is formed in the first quadrant of the coordinate, the shape of its cross section at right angle to the roller shaft is formed in a recessed circular arc shape with a single radius, and the center of the circular arc is set in the third quadrant of the coordinate and on the outside of a circle drawn with a radius r0 about O1.