In an image acquisition system, a distortion distribution is easily measured in a wide range. A standard image of magnetic domain of a sample serving as a standard is acquired by radiation of light using a standard external magnetic field which serves as a standard, a plurality of magnetic domain images are acquired in a state where an external magnetic field is applied while being changed, a plurality of subtraction images obtained by subtracting the standard image of magnetic domain from each of the plurality of magnetic domain images are acquired, a magnetization reversal area in which a magnetic domain is reversed is extracted from each of the plurality of subtraction images, and a composite image having a plurality of magnetization reversal areas is acquired by compositing the plurality of subtraction images each having the magnetization reversal area.