1. A method of determining static time phase of a coronary artery of a subject's beating heart using a magnetic resonance imaging (MRI) apparatus configured to generate a static magnetic field and a superimposed gradient magnetic field, to detect a plurality of magnetic resonance signals generated in the subject lying in the static magnetic field and induced by a radio-frequency magnetic field emanating from a transmitting coil, and to reconstruct an image from the plurality of magnetic resonance signals, the method further comprising: using said MRI apparatus to collect a time-series of N cine images of the coronary artery of the subject's heart, wherein an MRI data acquisition pulse sequence and imaged cross-section of the cine images cause the coronary artery to be visualized in the cine images as a high-level signal region that is rimmed with a closed loop low-signal region; encoding each image of the time-series of N cine images into a binary image; detecting in each of the binary images the closed loop of the coronary artery; determining a center of gravity of the closed loop for each image of the time-series of cine images on the basis of the detected closed loop; for N−1 images of the time series of N cine images, calculating motion-characteristic values indicating how much positions of the centers of gravity in the N−1 images deviate from the position of the center of gravity in another image of the time-series of N cine images; and extracting a specific time-series image whose motion-characteristic value is equal to or below a threshold.