A monitor image display method for a radiation-irradiation device that irradiates a radiation image recording medium with radiation to take a radiation image and is adapted to be capable of traveling, the radiation-irradiation device including a radiation source that generates radiation, a radiation source holding unit that is capable of at least setting the radiation source to a direction in which the radiation source irradiates the radiation image recording medium with radiation at the time of radiation irradiation and setting the radiation source to a vertically downward direction, characterised in that the device includes at least one optical camera that is capable of taking at least a front image showing a field of view on a front side in a traveling direction of the radiation-irradiation device and an irradiation-direction image showing a field of view in a radiation-irradiation direction at the time of radiation irradiation, and a monitor that displays the image taken by the optical camera, and the monitor image display method comprises: displaying the irradiation-direction image on the monitor in a case in which the radiation source is set to a direction other than a downward direction; displaying the irradiation-direction image on the monitor in a case in which the radiation source is set to the downward direction and a first condition is satisfied; and displaying the front image on the monitor in a case in which the radiation source is set to the downward direction and a second condition different from the first condition is satisfied. a radiation source that generates radiation, a radiation source holding unit that is capable of at least setting the radiation source to a direction in which the radiation source irradiates the radiation image recording medium with radiation at the time of radiation irradiation and setting the radiation source to a vertically downward direction, characterised in that the device includes at least one optical camera that is capable of taking at least a front image showing a field of view on a front side in a traveling direction of the radiation-irradiation device and an irradiation-direction image showing a field of view in a radiation-irradiation direction at the time of radiation irradiation, and a monitor that displays the image taken by the optical camera, and the monitor image display method comprises: displaying the irradiation-direction image on the monitor in a case in which the radiation source is set to a direction other than a downward direction; displaying the irradiation-direction image on the monitor in a case in which the radiation source is set to the downward direction and a first condition is satisfied; and displaying the front image on the monitor in a case in which the radiation source is set to the downward direction and a second condition different from the first condition is satisfied. displaying the irradiation-direction image on the monitor in a case in which the radiation source is set to a direction other than a downward direction; displaying the irradiation-direction image on the monitor in a case in which the radiation source is set to the downward direction and a first condition is satisfied; and displaying the front image on the monitor in a case in which the radiation source is set to the downward direction and a second condition different from the first condition is satisfied.