1. A discharge determination method for determining whether a discharge state where liquid is discharged from a nozzle or a non-discharge state where the liquid is not discharged from the nozzle is set, the discharge determination method comprising: performing imaging to obtain an image of an imaging field of view, wherein said discharged liquid flows down a flow-down path, and at least a part of said flow-down path is included in said imaging field of view; obtaining an index value which is larger in the discharge state than in the non-discharge state based on pixel values of pixels in an evaluation area including at least a part of said image; and determining that the image indicates the discharge state if the index value is larger than a reference value which is determined in advance while determining that the image indicates the non-discharge state if the index value is smaller than the reference value, the reference value being determined based on a histogram indicated by the index values obtained for the evaluation area of each of a plurality of images including the evaluation areas imaged at mutually different times, wherein when the histogram exhibits unimodality: if an evaluation value indicated by either the index value corresponding to the position of a peak or a value indicating the sharpness of the peak is smaller than a threshold value determined in advance, a value corresponding to a substantially maximum value of the index values in the histogram is set as the reference value; and if the evaluation value is larger than the threshold value, a value corresponding to a substantially minimum value of the index values in the histogram is set as the reference value; and wherein when the histogram exhibits bimodality, the index value corresponding to a valley between two peaks is set as the reference value.