A substrate processing apparatus includes a plurality of processing units and a gas supply unit. The plurality of processing units are stacked and arranged, and each configured to hold a substrate in a chamber and to process the substrate by a processing liquid, and the gas supply unit is provided for each of the processing units to supply a gas into each of the processing units. The gas supply unit includes an intake unit and an air supply unit. The intake unit takes in and purifies outside air, and the air supply unit configured to supplies a clean air purified by the intake unit into the processing units. In addition, the intake unit is arranged on a lateral side of the chamber, and is arranged on the same side face of the chambers between the stacked and arranged processing units.