Substrate processing apparatus and nozzle

After a discharge of a processing liquid is stopped, a position of a liquid surface within a nozzle can be observed. A substrate processing apparatus includes a substrate holding mechanism and the nozzle. The substrate holding mechanism is configured to hold a substrate. The nozzle is configured to supply the processing liquid to the substrate. The nozzle includes a pipe member and an observation window. The pipe member has a horizontal part and a downward part extended downwards from the horizontal part, and is configured to discharge the processing liquid from a tip end of the downward part. The observation window is provided at the horizontal part of the pipe member.