The invention relates to a substrate processing device which can measure the state of the processing liquid sprayed from a slit nozzle accurately. The invention is provided with a pipe (42) for extracting air at the secondary side of the slit nozzle (41). The pipe (42) is independent instead of directly communicating with the pipe (63) positioned at the first side of the slit nozzle (41). Besides, the pipe (42) is directly communicated with a stream (410) inside the slit nozzle (41). The pipe (42) is provided with a pressure sensor (413) so as to transfer the measure value to a control part (8). The corrosion resistance liquid is supplied to the slit nozzle (41) through a pump (61) and the pressure inside the pipe (42) at this time is measured through the pressure sensor (413).