A co-crystal composed of a diamide-based insecticidal-active ingredient and a neonicotinoid-based insecticidal-active ingredient is provided as a co-crystal which exhibits excellent insecticidal effects over a long period of time while reducing the occurrence of chemical damage on useful plants. The co-crystal is obtained by a method including heating and stirring of a suspension obtained by suspending the diamide-based insecticidal-active ingredient and the neonicotinoid-based insecticidal-active ingredient in a solvent such as water.