Interaction force between solid surfaces in aqueous solvent was investigated using scanning probe microscopy. Only short-range repulsive forces were observed in a poly(ammonium acrylate) (PAA) solution. Their range increased concomitantly with increase of the molecular weight of the PAA. In a PAA solution with 0.1–1 mM KCl, the PAA adsorbed onto the Si substrate. Furthermore, the interaction force was described by DLVO theory, and repulsive force is thought to originate from the steric hindrance of the PAA.