An illuminating device according to the present invention includes at least a first nitride-based semiconductor light-emitting element and a second nitride-based semiconductor light-emitting element, in which: the first nitride-based semiconductor light-emitting element and the second nitride-based semiconductor light-emitting element each include a semiconductor chip; the semiconductor chip includes a nitride-based semiconductor multilayer structure 45 formed from an AlxInyGazN (x+y+z=1, x≧0, y≧0, z≧0) semiconductor, and the nitride-based semiconductor multilayer structure 20 includes an active layer region 24 having an m-plane as an interface; the first nitride-based semiconductor light-emitting element and the second nitride-based semiconductor light-emitting element each emit polarized light from the active layer region 24; and, when the polarized light emitted from the first nitride-based semiconductor light-emitting element and the polarized light emitted from the second nitride-based semiconductor light-emitting element have wavelengths λ1 and λ2, respectively, and thicknesses of the semiconductor chips 45 of the first nitride-based semiconductor light-emitting element and the second nitride-based semiconductor light-emitting element are given as d1 and d2, respectively, the following relations are satisfied: λ1