The invention provides a device to inspect an illumination optical device and a method to inspect an illumination optical device that make it possible to efficiently inspect illumination optical devices and to control manufacturing cost. A lens array inspecting device is provided with a light source device to emit a parallel luminous flux, lens array holders to retain lens arrays, which are test objects that split the parallel luminous flux into a plurality of partial luminous fluxes, and a ground glass on which the optical images of the luminous fluxes emitted through the lens arrays are projected. On the ground glass, a parting frame appropriate to a design illumination region is formed. Therefore, whether the lens arrays are defective or non-defective can be determined by checking whether the optical images projected onto the ground glass include the area of the parting frame.