An ultraviolet irradiation device includes a treatment vessel, an ultraviolet irradiation member, and a support member. The treatment vessel has a water inlet and a water outlet and through which water to be treated as a treatment target flows in a first direction from the water inlet toward the water outlet, the treatment vessel receiving the water to be treated through the water inlet and discharging the water to be treated through the water outlet. The ultraviolet irradiation member is provided inside the treatment vessel along a second direction crossing the first direction and which irradiates the water to be treated flowing through the treatment vessel with an ultraviolet ray. The support member is provided inside the treatment vessel along the second direction with both end portions of the support member being firmly fixed to wall surfaces of the treatment vessel.