1. A magnetic resonance imaging apparatus comprising: an imaging unit which includes a transmission unit configured to irradiate a high-frequency magnetic field onto an examination target and a reception unit configured to receive a nuclear magnetic resonance signal from the examination target; an arithmetic unit which processes the nuclear magnetic resonance signal acquired by the reception unit and performs an arithmetic operation including image reconstruction; and a control unit which controls imaging by the imaging unit, wherein the transmission unit includes a transmission coil having multiple channels, and the control unit has an image acquisition sequence in which an image is acquired by irradiation with one channel or a combination of two or more of the multiple channels, and an irradiation magnetic field distribution measurement sequence in which an irradiation magnetic field distribution upon irradiation with all of the multiple channels is measured, and the arithmetic unit includes a first irradiation magnetic field distribution calculation unit which calculates the irradiation magnetic field distribution of all of the multiple channels using data acquired in the irradiation magnetic field distribution measurement sequence, and a second irradiation magnetic field distribution calculation unit which calculates the irradiation magnetic field distribution of each channel of the multiple channels using multiple pieces of image data acquired in the image acquisition sequence and the irradiation magnetic field distribution of all of the multiple channels calculated by the first irradiation magnetic field distribution calculation unit, and wherein the control unit has additionally a second image acquisition sequence in which an image of an object is acquired, and the arithmetic unit includes a shimming unit which calculates a set of amplitude and phase of a high-frequency magnetic field to be irradiated in the second image acquisition sequence, using the irradiation magnetic field distribution calculated by the second irradiation magnetic field distribution calculation unit, for each channel of the multiple channels.