The present invention provides a process for preparing a diester compound of the following general formula (1), having a dimethylcyclobutane ring, wherein R1 and R2 represent, independently of each other, a monovalent hydrocarbon group having 1 to 10 carbon atoms, the process comprising reacting a dimethylcyclobutanone compound of the following general formula (2), wherein R1 is as defined above, with a phosphonic ester compound of the following general formula (3), wherein R2 and R3 represent, independently of each other, a monovalent hydrocarbon group having 1 to 10 carbon atoms, to produce the diester compound (1), having a dimethylcyclobutane ring.