PURPOSE: To obtain an accurate MTF value, by aligning the phases of the distributions of intensities so that the positions of the maximum values of the distributions of the intensities of line images, which are taken for plurality of times, agree to each other, and averaging the intensities of the line images.

CONSTITUTION: One of the signals from a CCD array 6, in which line images are formed, is selected by a multiplexer MPX8, and received by an RAM10 through an AD converter 9. When the receiving times are three times, three line image data (a)W(c) are received. One line image data (a) is retrieved out of the RAM10. Sensitivity and dispersion are corrected by a correcting circuit 11. Then, the intensity of the line image is detected by an MAX routine 12. A specified number of memories before and after the maximum value only are stored in an adding circuit 13. The same processing is performed for the remaining data (b) and (c). From the adding circuit 13, the sum or the average value of the three line image intensity distributions, whose phases are aligned, is outputted. Then, Fourier transformation is performed by a CPU7, and a highly accurate MTF is measured.