A power transmission in which a driven body and a drive body for driving the driven body are rotated in the same direction and are coupled through a coupling portion, transmission of torque from the drive body is interrupted when the driving load of the driven body exceeds a predetermined level, and the coupling portion is constituted by combining members different from each other of a positive torque transmission member for transmitting torque in the direction of forward rotation and interrupting transmission of torque from the drive body by its own fracture when the driving load of the driven body exceeds the predetermined level and a negative torque transmission member capable of transmitting torque in the direction of reverse rotation. At a contact portion between the positive torque transmission member and the negative torque transmission member, interposed is a surface layer composed of a different kind of material from a material composing the positive and negative torque transmission members. Reliability and durability of the power transmission can be improved by preventing occurrence of melt-adhesion, abrasion or rust at the contact portion between the members constituting the coupling portion.