1. A nuclear magnetic resonance imaging device comprising control and computation means, wherein radio frequency pulses and magnetic fields are applied to an object in time relationships designated by said control and computation means to generate nuclear magnetic resonance signals and an image of said object is obtained by using said nuclear magnetic resonance signals; wherein said device further comprises an operation console having input means for setting input information desired for said device; said control and computation means being adapted to store relaxation times T1, T2, nuclei density .rho., pulse sequences suitable for obtaining a calculated image, and scan conditions, including scan parameters; said scan conditions being determined externally or internally by the following procedure and being used as scan conditions; said scan conditions being read out from said control and computation means under designation from said operation console and said image being taken in accordance with said scan conditions; wherein said procedure comprises selecting and determining a pulse sequence to be used; determining a theoretical equation of signal intensity in accordance with said pulse sequence selected in a manner as described above and with values for T1, T2, .rho., at one or more points, as representative points within a range of values T1, T2, .rho., to be noted, and determining a variance or standard deviation of a calculated image for T1, T2, .rho., as a function of scan parameters from said theoretical equation and variance in values for T1, T2, .rho. and original image; and determining scan parameters with which a total sum of said variance or standard deviation of desired calculated image, or a total sum of weighted variance or standard deviation of said desired calculated image, among said calculated images for T1, T2, .rho., takes a local minimum value, and using said scan parameters as optimum scan conditions.