1. A magnetic resonance imaging apparatus that applies a gradient magnetic field to an imaging region, generates k-space data including a plurality of matrix elements by sampling a nuclear magnetic resonance signal acquired from the imaging region and reconstructs image data based on the k-space data, comprising: a gradient magnetic field power supply configured to apply the gradient magnetic field to the imaging region by supplying gradient magnetic field current to a gradient magnetic field coil under an imaging sequence; a gradient magnetic field calculating unit configured to calculate a waveform of the gradient magnetic field current based on conditions of the imaging sequence, and calculate a waveform of a gradient magnetic field in a readout direction based on a mutual inductance by which the gradient magnetic field coil causes mutual induction and the waveform of the gradient magnetic field current; and a regridding processing unit configured to generate or rearrange the k-space data by sampling at unequally-spaced intervals, so that a part of the nuclear magnetic resonance signal acquired during a time span in which a time integral value of intensity of the gradient magnetic field in a readout direction is non-linear and each time integral value up to a sampling period corresponding to each of the matrix elements becomes equally-spaced.