PURPOSE: To correct the actual size of the image of an object to be detected by forming the images of both the reference samples and object to be detected which are disposed in vertical and horizontal positions, correcting the size of the image of the object by the distances between the reference samples and displaying scales.

CONSTITUTION: The reference samples 22, 23, 24, 25 in the vertical and horizontal directions are provided to a nuclear magnetic resonance calculating tomographic device 1. All these samples are the water or aq. soln. contained in pipes each consisting of a non-magnetic material. The object to be detected is disposed in a circular partition wall 26 and is converted to the image by two-dimension Fourier transform. The image 4a of the object to be detected and the images 22aW25a of the reference samples are displayed. The aspect ratio of the image 4a is corrected by utilizing the distances between the images 22aW25a to form a reference image 4b which is then displayed together with the scales 27, 28. Since the image is corrected by utilizing the actual sizes between the reference samples, the image having the exact aspect ratio is obtd. and the point-to-point measurement within the image with high accuracy is made possible.