A substrate processing apparatus has a labyrinth around a processing liquid nozzle above a nozzle gap, and a seal gas is supplied to the labyrinth to seal the nozzle gap from an external space. Consequently, the entry of the atmosphere of the external space into a processing space through the nozzle gap can be suppressed. An opposing-member flange part of a top plate has a first uneven part on the upper surface, and a holder body of an opposing-member moving mechanism has a second uneven part on the lower surface. The labyrinth is formed by raised portions of one of the first and second uneven parts being disposed within recessed portions of the other of the first and second uneven parts with a gap therebetween only when the top plate is located at a second position (i.e., the processing space is created). This achieves flattening of the substrate processing apparatus.