A radiation image capturing system detects the position of a radiation detecting cassette disposed below a patient and the position of a radiation source for emitting a radiation, based on the differences between the propagation times of radio waves emitted from an antenna device to an image capturing apparatus and the radiation detecting cassette. Based on the detected positions, the relative positions of the image capturing apparatus and the radiation detecting cassette are calculated, and then compared with each other by a position determining unit to judge how the image capturing apparatus is positioned with respect to the radiation detecting cassette. If the image capturing apparatus is not positioned in head-on facing relation to the radiation detecting cassette, then a warning is issued, and an actuating mechanism moves the image capturing apparatus to an appropriate position.