A coating agent is capable of forming a coating film that has enhanced adhesion and adherence to a substrate, and enhanced water resistance as well and composed mainly of an aqueous material. A process of forming a coating agent uses the coating agent, a primer treatment process uses the coating agent, a process of doing repairs to concretes uses the coating agent, and a process of laying down roads uses the coating agent. The coating agent is composed mainly of a polyphenol derivative and containing a polymerizing agent, and has a pH of 9 or less. The polymerizing agent contains a compound having two or more functional groups selected from the group of an amino group and a mercapto group per molecule. In the process of forming a coating film, the coating agent is applied onto a substrate in an alkaline environment having a pH of greater than 9.