Whether or not a liquid is discharged from a nozzle is determined from an image. An index value larger in a discharge state than in a non-discharge state is calculated from an evaluation area including a path of the liquid. The discharge state or the non-discharge state is determined according to a relationship between the index value and a reference value. The reference value is determined according to a histogram of index values obtained from a plurality of images collected in advance. When an evaluation value indicating a peak position or the sharpness of a peak is smaller than a predetermined threshold value in the histogram, the collected images are assumed to show the non-discharge state. When the evaluation value is larger than the threshold value, the collected images are assumed to show the discharge state. Thus, the reference value is set for the discharge state and the non-discharge state.