To provide a rubber composition for a belt having excellent adhesive properties, the belt for power transmission, and a toothed belt.

This rubber composition for the belt is used in the formation of either one or both of an adhesive layer between a rubber and a core wire, and that between the rubber and a canvas, of the belt for power transmission having the rubber, the core wire, and the canvas, and comprises 1-8 pts.mass organic peroxide and 0.5-8 pts.mass N-N'-m-phenylenedimaleimide used as a co-crosslinking agent compounded to 100 pts.mass of a compounded matter comprising a composite polymer which is compounded with a hydrogenated nitrile rubber and a metal unsaturated carboxylate in a mass ratio of 20/80-70/30 and a hydrogenated nitrile rubber compounded in a mass ratio of 10/90-60/40 to it. This belt for power transmission has an adhesive layer using the rubber composition for the belt, and the toothed belt has an adhesive layer using the rubber composition for the belt and a back part.