A radiation image capturing system includes a radiation image capturing apparatus, one or more radiation generation apparatuses and a calculation unit. The radiation image capturing apparatus includes a control unit which controls a scan driving unit and a readout integrated circuit of the radiation image capturing apparatus since before start of irradiation of the radiation image capturing apparatus and detects the start of the irradiation when data read out by the readout integrated circuit is equal to or more than a threshold. Before radiation image capturing, the calculation unit calculates a maximum body thickness for each of the radiation generation apparatuses on the basis of the data read out in the radiation image capturing apparatus irradiated by the radiation generation apparatus, the maximum body thickness up to which the control unit can detect the start of the irradiation through a subject.