Anti-wear properties and tooth life in a toothed belt are improved by taking advantage of heat hardening of a treating agent used to treat the tooth cloth. The tooth cloth is treated with a resorcinol formaldehyde latex solution containing a mixture, at a weight ratio from 1:5 to 1:15, of a resorcinol formaldehyde solution containing resorcinol and formaldehyde at a molecular ratio from 1:1 to 1:3 and a latex solution containing carboxyl modified nitrile butadiene rubber and carboxyl modified hydrogenated nitrile rubber at a weight ratio from 4:6 to 1:9, and further containing carbon at a weight ratio of 1 to 10%. The solid content adhering onto the tooth cloth is from 5 to 30 weight %.