PURPOSE: Titled pretreating agent which does not necessitate sanding prior to bonding thereby making it possible to carry out bonding in sanitary environment, comprising a composition having a resin component consisting of an unsaturated polyester, an ethylenically unsaturated monomer, a cure accelerator and a polyisocyanate.

CONSTITUTION: The pretreating agent for bonding glass fiber-reinforced unsaturated polyester resin comprising a composition containing as a resin component, the following components: (A) 5W50wt% of an unsaturated polyester, (B) 5W50wt% of an ethylenically unsaturated monomer, (C) 0.1W5.0wt% of a cure accelerator and (D) a polyisocyanate resin. As a polyhydric alcohol used in the production of component A, a hydrogenated bisphenol A or the like is used. As (B) there is used styrene or a (meth)acrylate ester. As (C) there is used a metallic soap such as lead naphthenate or potassium octanoate, and as (D) there is used toluylene diisocyanate; 4,4'-methylene diphenyl diisocyanate, etc.