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 two or more channels, the control unit has an image acquisition sequence in which, for multiple channels which are the whole or a part of the transmission coil, an image is acquired by irradiation with one channel or a combination of two or more 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.