A "capillary condensation" in liquid phase was analyzed theoretically and experimentally, in which a solute in solvent could be separated out as a liquid into small pores by capillary effect. The "liquid-phase capillary condensation" is defined as capillary phase separation, The separated phase is counted as "adsorbed" amount on porous materials. The relation between the concentration and a critical radius of the phase separation was derived. A new method to estimate liquid-phase adsorption isotherms of adsorbates with limited solubility from pore-size distribution of solids is based on this concept. The amount absorbed consists of the separated phase and adsorption on surface expressed with a single parameter. While the surface adsorption alone could not explain measured isotherms, this model showed good agreements between experimental and estimated isotherms for adsorption of four kinds of aromatic compounds from aqueous solution onto three categories of porous solids to demonstrate the significance of the capillary phase separation. The applicability of the method to solid solutes is also discussed.