To provide a weak solvent-type undercoating composition having high sealing properties against a rusty surface, exhibiting excellent rust preventing properties and overcoatability; and a coating method using the undercoating composition.

The undercoating composition contains a urethane-modified soluble type epoxy resin (A) having a number average molecular weight of 500-10,000 and obtained by reacting a polyisocyanate (p) having an allophanate structure, obtained by reaction between a 1-12C monoalcohol and an isocyanate compound, with a hydroxy group-containing soluble type epoxy resin (q) such that the NCO/OH equivalent ratio is 0.02-0.5, an amine compound (B), and a rust-preventing pigment (C).