An water-based surface treatment agent used for the copper on a printed wiring board (PWB). Chemical layer formed on PWB shows excellent heat-resistance and moisture-resistance; therefore, the treated PWB preserves excellent soldability for a long time. The water-based surface treatment agent contains, as an active ingredient, a 2-arylimidazole compound represented by the following formula, ##STR1## wherein R is a hydrogen atom or a methyl group, R1 and R2 are hydrogen atoms, lower alkyl groups or halogen atoms, and R3 and R4 are hydrogen atoms, lower alkyl groups, halogen atoms, lower alkoxy groups, di-lower alkylamino groups, cyano groups or nitro groups, ##STR2## wherein R is a hydrogen atom or a methyl group, and R1 and R2 are hydrogen atoms, lower alkyl groups or halogen atoms, but excluding the case where R, R1 and R2 are all hydrogen atoms.