Ultraviolet curing systems are used in various fields of industry. Two types of irradiators are commonly used in these curing systems: the parabolic reflector type and the ellipsoidal reflector type. Although the parabolic reflector irradiator is necessarily even, the illuminance distribution does not become even. This study proposes an irradiated plane illuminance calculation method for the parabolic reflector type irradiator. The validity of the calculation method was verified experimentally. In addition, it was clarified that the irradiated plane’s illuminance distribution can be made even by varying the reflectance characteristics of sections of the parabolic reflector.