To provide a substrate processing apparatus which homogenizes and stabilizes the concentration of an organic solvent in an atmosphere containing an organic solvent such as IPA(isopropyl alcohol) or the like.

Gas flow of IPA vapor having flowed from a gas inlet tube 56 through a gas inlet opening 33 is decelerated rapidly by wide buffer spaces 37 so as not to maintain the form of gas flow, and changes into gas that makes just ordinary motion by diffusion. The IPA vapor having changed into ordinary gas is pushed up by the IPA gas newly flowed in, and is pushed out divergently from a gas supply opening 35. With this constitution, the amount of the IPA vapor per unit area flowed out from the gas supply opening 35 becomes uniform. Besides, since the cross sectional area of the gas supply opening 35 is much larger than the cross sectional area of the gas inlet opening 33, the IPA gas does not form gas flow of ejecting from the gas supply opening 35, and the atmosphere containing the IPA vapor in the upper part of the processing chamber 20 has no danger of being disturbed.