PURPOSE: To eliminate the influence of noises for comparison of characteristic curves and to extract an organic area of an MRI image by deciding the coincidence or discordance of two characteristic curves based on a membership function which applies the absolute value of the difference between both curves as a variable.

CONSTITUTION: A memory has a function which applies the absolute value of the difference between a reference curve obtained from the characteristic curve of an echo signal presumed by a spin echo method owing to a multi-error and a reference characteristic curve of an extracted organ as a variable. At the same time, the absolute value δ is obtained for the difference between the reference curve and the actually measured reference characteristic curve of the extracted organ. Then the membership function value μ is obtained from the value δ. The measured echo signals area available to all degrees and therefore an evaluation function is obtained from the function value of all degrees. Then it is decided that the echo signal generating part is identical to the extracted organ as long as the evaluation function value is kept within a prescribed range. Thus an organ is extracted in an image, and an echo signal is extracted at each point of the relevant area. The mutual product of membership function values of all degrees or the sum total of function values of all degrees is used to the evaluation function.