A friction drive device includes a first roller with a support part, a second roller, and a cam member to support them. The second roller receives press force from the first roller at a contact point of them so as to transmit mechanical power therebetween. The cam member has a cam formed with a cam slope on which the support part of the first roller is pressed by reaction torque from the second roller that is caused by transmission torque applied from the first roller. The cam slope is formed to have an angle with respect a tangent line at the contact point so that the first roller is pressed by reaction force of the cam to apply the press force proportional to the transmission torque to the second roller.