An apparatus which can accurately measure surface forces between solid surfaces in an atmosphere-controlled chamber has been developed. The surface forces between crossed-cylindrical mica surfaces were measured in undersaturated vapors of various liquids (hydrocarbons, alcohols, and water) using this apparatus. The surface forces changed gradually with an increase in the relative vapor pressure of liquid from those under dry conditions to macroscopic meniscus forces. To investigate the formation of the meniscus bridge between solid surfaces, the authors introduce the critical relative vapor pressure, (p/p(s))(c), at which the surface adhesive force approaches the macroscopic meniscus force. The Kelvin radius at this critical relative vapor pressure was on the nanometer level and was found to be dependent on the structure, molecular weight, and electrostatic nature of the molecules.