A three-dimensional periodic structure exhibiting a complete photonic band gap in a wide wavelength range and being readily produced, as well as a functional element including the same, is provided. In the three-dimensional periodic structure exhibiting a photonic band gap according to the present invention, layers composed of a plurality of square columns spaced apart by a predetermined interval are stacked sequentially with additional layers therebetween, rectangular parallelepipeds contained in the additional layers are disposed at the positions corresponding to the intersections of the square columns, and 1.21.ltoreq.W1/W.ltoreq.2.39 and W/W1