The radiation-irradiation device includes: a radiation generating unit that generates radiation; a collimator unit that controls an irradiation range to be irradiated with the radiation generated by the radiation generating unit; and an interval ensuring unit that includes a contact member being in contact with a subject to be irradiated with the radiation in a case in which a distance between the radiation generating unit and the subject is shorter than a preset distance, and ensures an interval between the radiation generating unit and the subject. The interval ensuring unit is detachably mounted on the collimator unit and is capable of being mounted at different rotational positions about an axis, which passes through a center of a diaphragm of the collimator unit and extends in the direction of an optical axis of the radiation, as a central axis.