A portable radiation imaging system is provided which is capable of always performing imaging in a state where the tilt of the radiation emitted from a radiation source in relation to the detection surface of an image detection device is substantially perpendicular. The system is equipped with a tilt adjustment means that makes the tilt of the radiation in relation to the detection surface of a radiation image detection device substantially perpendicular by changing the tilt angle of a radiation source based on an angular signal representing the degree of tilt of the radiation in relation to said detection surface output by an angular signal output means, and a command means that generates an exposure command to the radiation source when the tilt of the radiation to be emitted from said radiation source in relation to the detection surface of said radiation image detection device is substantially perpendicular.