In this study, inverse opal photonic crystals (IOPCs) with mesopores (diameter, 2–50 nm) were fabricated by the optimization of the size ratio of large to small polystyrene (PS) nanoparticles used as the template of the IOPCs and the concentration of the precursor forming the structure of the IOPCs. The generation of cracks and defects on the obtained IOPCs was avoided by using the co-assembly method, which is a self-organized fabrication method for IOPCs using nanoparticles of a polymer such as PS as a template in a precursor solution. 4-Amino-3-pentan-2-one (Fluoral-P) was infiltrated into the IOPCs, and the edge of the photonic band gap of an IOPC fabricated using 350 and 50 nm PS nanoparticles (50 nm-350IOPC) was matched with part of the fluorescence wavelength range of 3,5-diacetyl-1,4-dihydro-2,6-lutidine (DDL), which is a substance emitting fluorescence resulting from the reaction of Fluoral-P with formaldehyde. It was found that introducing mesopores into 350IOPC with infiltrated Fluoral-P was effective for enhancing the signal in formaldehyde detection.