Abstract: A substrate processing apparatus is provided with a spin base for rotationally holding a substrate, an atmosphere cutoff plate for cutting off a top surface of the substrate from the outer atmosphere, a splash guard for receiving a processing solution spun off from the substrate or the like, a guard up-and-down moving mechanism for vertically moving the splash guard. In performing processing with a processing solution while rotating a substrate, the splash guard is disposed by the guard up-and-down moving mechanism so that the level of a top surface of the splash guard should not be higher than the level of a top surface of the atmosphere cutoff plate and it is thereby possible to prevent the atmosphere above the atmosphere cutoff plate from being involved in the recovery duct through rotation of the atmosphere cutoff plate.