1. A magnetic resonance imaging apparatus, comprising, a static magnetic field forming unit for forming a static magnetic field in a space where a subject is placed, a gradient magnetic field applying unit for applying a gradient magnetic field to the subject, a radio frequency magnetic field applying unit having a transmission coil with multiple channels and applying a radio frequency magnetic field to the subject, a signal receiving unit for receiving a nuclear magnetic resonance signal generated from the subject, a measurement control unit for controlling according to a predetermined imaging sequence, operations of the gradient magnetic field applying unit, the radio frequency magnetic field applying unit, and the signal receiving unit, thereby performing measurement, and a transmitting sensitivity shimming preparing unit for calculating inter-channel phase difference and amplitude ratio of RF pulses applied from the channels respectively, so as to correct inhomogeneity of a transmitting sensitivity distribution of the transmission coil, the transmitting sensitivity shimming preparing unit further comprising, a transmitting sensitivity measuring unit for repeatedly executing an imaging sequence to which a prepulse is added while applying the prepulse at different flip angles, thereby acquiring multiple reconstructed images for the respective flip angles of the prepulse, with respect to each of the channels of the transmission coil, a transmitting sensitivity distribution calculating unit for determining a reference image from the multiple reconstructed images, calculating a subtraction image between the reference image and each of the reconstructed images other than the reference image, with respect to each of the channels of the transmission coil, and calculating the transmitting sensitivity distribution by using the subtraction images being calculated, a phase difference and amplitude ratio calculating unit for calculating independently, inter-channel phase difference and amplitude ratio of the RF pulses transmitted to the respective channels of the transmission coil, based on the transmitting sensitivity distribution being calculated, and a phase difference and amplitude ratio setting unit for setting the phase difference and the amplitude ratio being calculated, to the measurement control unit, and the measurement control unit controlling the radio frequency magnetic field applying unit to apply the RF pulses with the phase difference and the amplitude ratio being set, from the respective channels of the transmission coil.