A concentration correcting method includes: forming N test patterns at different positions in a moving direction by repeatedly performing a dot line forming process of forming a dot line in a line region on a test medium in which a plurality of unit areas is arranged in the moving direction by ejecting liquid from nozzles direction and a transport process of transporting the test medium in a transport direction; performing on the N test patterns a process of calculating a concentration correction value, which is used to correct concentrations of the unit areas in a print image; and correcting the concentrations of the unit areas in the print image by the use of interpolated correction values obtained using a linear interpolation method, two concentration correcting values acquired from the two test patterns, and positions of the unit areas in the moving direction.