An alicyclic diepoxy compound (A) represented by formula (I) is produced in high purity and high yield at low cost, by epoxidizing the corresponding alicyclic diolefin compound with an organic percarboxylic acid. The curable epoxy resin composition has high reactivity for various curing agents, low viscosity, and excellent workability. A cured product thereof shows useful physical properties for uses in coatings, inks, adhesives, sealants, and encapsulants, etc. It is of extremely high quality as an epoxy resin composition for the encapsulation of electronic parts. A stabilizer for an electrical insulating oil (the alicyclic diepoxy compound or an electrical insulting oil containing the compound) is low in acid value, and the stabilizer improves the properties of the insulating oil. A cured product obtained by curing a casting epoxy resin composition for electrical insulation has excellent properties such as high bending strength, high Tg, and low permittivity.