Provided is an image processing apparatus capable of assuredly finding a lung field that appears in an image even if a subject image appears in a radiation image in a rotated manner and assuredly performing a contrast adjustment with excellent visibility for the lung field. The image processing apparatus is provided with a sideways determination means 13 configured to determine that the subject image is turned sideways in the original image P0 when a minimum point P of a profile generated by summing or averaging pixel values belonging to each pixel column of the original image P0 for each pixel column is away from the center position of the subject image. The original image P0 determined as being turned sideways by the sideways determination portion 13 is rotated by the image rotation portion 16 and used for searching of the lung field.
