A method of and an apparatus for simultaneous sizing and drying a complete number of warps of filament yarns required for weaving of cloth. During sizing, a warp sheet having high yarn density is put to uniform arrangement state using a first slant reed and a guide roller, and the warp sheet in the uniform arrangement state passes through a sizing solution and is squeezed at high pressure. The sized yarns are separated using dividing rods and a second upright reed before preparatory drying, and separated using a third upright reed and dividing rods after the preparatory drying, thereby the sized yarns in separated state in vertical and lateral directions pass through hot air during the preparatory drying. Yarns separated after the preparatory drying are put to uniform arrangement state using a fourth slant reed and a guide roller, and the yarns in the uniform arrangement state are laid over heating cylinders during finishing drying.