A printing apparatus including a discharge head that performs scanning movement with respect to a print medium and discharges ink to a surface of the print medium, a signal output unit which includes a reference scale in which optical density changes stepwise along a scanning movement direction of the discharge head and a reading unit that optically reads the reference scale and outputs a signal according to the optical density and in which the reference scale and the reading unit move relatively with each other along with the movement of the discharge head, and a stain detection unit that detects stain of the signal output unit based on a detection result of a strength level of the signal.