1. A nuclear magnetic resonance imaging apparatus comprising: a transmission coil having a plurality of channels that respectively transmit high frequencies (RF) to an object; an RF shimming section determining at least one of an amplitude and a phase of the high frequencies to be transmitted respectively to a plurality of the channels as a high frequency magnetic field condition based on an objective function and a restriction condition so as to improve homogeneity of a high-frequency magnetic field (B 1 ) distribution that the transmission coil generates and to reduce a specific absorption ratio (SAR) of the object; and a high-frequency signal generator generating the high frequencies for each of the channels based on the high frequency magnetic field condition, wherein the RF shimming section determines objective function parameter values for each of the channels for setting the objective function according to contribution to the specific absorption ratio, and wherein the objective function parameters includes a distribution weight (w), a weight (w amp ) of each of the channels, and an exponent (k), the objective function is a linear combination in which a term showing reduction of the specific absorption ratio and a term showing improvement of the homogeneity are weighted with the distribution weight (w), the term showing the reduction of the specific absorption ratio is formed by performing weighted addition for the exponential (the k-th power) of the amplitude of the high frequencies for each channel with the weight (w amp ), and the RF shimming section determines the high frequency magnetic field condition as a solution that optimizes the objective function.