The present invention for a given substrate processing apparatus and a substrate processing method for performing processing in a state in which supporting a substrate in a substantially horizontal, yet achieve the effect of blocking with the blocking plate processing and similar, the technology for size reduction of more devices for its object to provide. As a solution, and installing the gas injection head 200 in approximately the center above the substrate (W). Gas also injects nitrogen gas introduced from the inlet 291, from the injection port 293 of the slit-shaped space via a buffer (BF) of the inner. Thus by the top of the substrate, is formed with a substantially radial gas flow in the isotropic On the other hand, in the horizontal direction is the direction in which the jet limited in the vertical direction. Therefore, the dust (D) or mist (M), including the surrounding substrate is not being flowed out to the outside edges attached to the substrate (W). Since the gas injection head 200 is not necessary to can be made smaller than the diameter of the substrate (W), addition to or retracted from the surface of the substrate rotation, the device can be configured compact. A substrate processing apparatus, the substrate processing method, miniaturization, dust, mist,