1. A magnetic resonance imaging method, which comprises the steps of: repeatedly applying a radio frequency pulse, a slice selecting gradient magnetic field pulse along a first axis, a phase encoding gradient magnetic field pulse along a second axis perpendicular to said first axis and a frequency encoding gradient magnetic field pulse along a third axis perpendicular to said first and second axes to an object to be imaged which is placed in a static magnetic field while varying an amount of phase encoding given by said phase encoding gradient magnetic field pulse to thereby generate nuclear magnetic resonance signals from a slice of said object normal to said first axis; changing a thickness of said slice corresponding to said amount of phase encoding given by said phase encoding gradient magnetic field pulse so that said nuclear magnetic resonance signals are obtained from said slice which has been changed in thickness; and producing an image of said object in accordance with said nuclear magnetic resonance signals obtained from said slice which has been changed in thickness.