To provide a friction transmission device in which a life and power transmission efficiency can be increased by approximately making constant the ratio of a transmission force to a pressing force without relying on the magnitude of the transmission force while facilitating the processing of a cam face and the shape inspection of a finished product.

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 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 a circumference with a radius r0 about O1 or in the vicinity thereof.