1. Nuclear magnetic resonance diagnostic apparatus wherein a uniform static magnetic field is generated, a first gradient magnetic field is superimposed on said static magnetic field in the direction of said static magnetic field, a second gradient magnetic field is superimposed on both said static magnetic field and said first gradient magnetic field in a direction perpendicular to said magnetic field direction, and the gradient direction of said second gradient magnetic field is rotationally shifted through a plurality of steps substantially equal to 360 degrees divided by some unit index angle rotated to acquire a tomographic image on the basis of nuclear magnetic resonance signals due to specified atomic nuclei existing within a slice of an object, the apparatus comprising: coil means for generating said second gradient magnetic field; means for controlling the operation of said coil means to rotationally shift in angular steps the gradient direction of said second gradient field in a slice of an object to be diagnosed where said angular step is a multiple of said index angle in a sequence to assume all of the gradient direction positions of said second gradient field; a memory device for storing nuclear magnetic resonance signals for each gradient direction of said second gradient magnetic field; a rough image reconstructor for reconstructing a rough tomographic image on the basis of nuclear magnetic resonance signals acquired during a rotation of the gradient direction of said second gradient magnetic field; a rough image display for depicting said rough tomographic image; a final image reconstructor for reconstructing a final tomographic image on the basis of nuclear magnetic resonance signals corresponding to all of the expected rotational shift positions acquired during a plurality of rotations; and a final image display for depicting said final tomographic image.