A radiation irradiation device is provided that includes: a metal target that emits bremsstrahlung X-rays as a radiation beam due to irradiation with an electron beam; a radiation shielding member that includes a slit-shaped radiation passage portion and that is disposed downstream of the metal target in the radiation beam emission direction and is disposed such that a portion of the radiation beam passes through the radiation passage portion and the radiation beam incident to regions other than the radiation passage portion is blocked; and an electron beam generating device that irradiates, onto the metal target, an electron beam such that a diameter at a generation point of the emitted radiation beam is smaller than a length of an entry portion of the radiation passage portion along a length direction of the entry portion.