Disclosed are an evaporator, an evaporation method, and a substrate processing apparatus, which can increase the concentration of generated vapor of an organic solvent and efficiently heat the organic solvent. The evaporator includes a fluid tube, a liquid organic solvent supply device for supplying the organic solvent liquid to one end of the fluid tube, and heating units for heating the fluid tube. The fluid tube has a cross section that increases from the one end to the other end. When the organic solvent liquid supplied to one end of the fluid tube is heated, the organic solvent vapor is discharged from the other end of the fluid tube. The substrate processing apparatus includes the above-described evaporator.