Disclosed herein is a radiation imaging apparatus and radiation-imaging-apparatus-based nuclear medicine diagnosis apparatus having a collimator in which a plurality of rectangular through-holes are arranged in a grid pattern and separated by septa is rotated through a predetermined angle as viewed from above in relation to the layout of a plurality of rectangular detectors that are arranged in a grid pattern. The predetermined angle ranges from 20 to 70 degree and more preferably from 30 deg to 60 deg. With this configuration, the influence of sensitivity variations (moire patterns) that are included in an image picked up due to interference with a collimator when pixel type detectors are used is eliminated.