A waiting time Ts is decided on the basis of ink type information and ink feed amount information, referring to waiting time table A (Step 6). Along with this, a dried printing paper is carried to and stopped in a waiting area 9 (Step 7), a time period T is measured after stopping, and measurement processing in the next step is delayed until the time period T reaches the waiting time Ts. Then, when the time period T reaches the waiting time Ts, a measuring instrument 10 is used to measure a density of a patch printed on the printing paper (Steps 8 and 9).