1. An image discrimination method for discriminating presence or absence of data of a plain image in data of an radiographed image radiographed with a sensor, the plain image being an image of an area to which a radiograph is directly irradiated into the sensor not through an object, comprising: a step of obtaining a characteristic value representing the data of the radiographed image; a step of extracting an image area corresponding to data of an radiograph irradiation area in the data of the radiographed image, a step of calculating occurrence frequency of pixels having pixel values more than the characteristic value obtained in said step of obtaining a characteristic value, among pixel values in a peripheral region including an edge part of the image area within the image area correspond to the radiograph irradiation area; and a step of discriminating presence or absence of data of a plain image in the data of the radiographed image based on the occurrence frequency.