A cylinder barrel inside surface treatment apparatus for providing a uniform treatment finish is disclosed. The treatment apparatus includes a pallet on which a cylinder block is placed with a gasket surface faced upward. A top opening of a cylinder barrel of the cylinder block is covered by a cover member having a concave portion communicating with the top opening. A nozzle having treatment solution spray holes is attached to the cover member. The treatment solution is sprayed toward a wall of the concave portion from the treatment solution spray holes and flows along the wall from a top end to a bottom end of the cylinder barrel inside surface.