Dicyclopentadiene → 2,3,3a,4-Tetrahydro-1H-indene

A reaction is carried out with a system consisting of one decomposition reactor, one Diels Alder reactor and two distillation column. A mixture of a starting dicyclopentadiene material containing tetrahydroindene and a solvent (weight ratio of 2:5) is continuously fed to the decomposition reactor, and a reaction is carried out at the same temperature, pressure, reaction time and quantity of charged solvent as in Example 4. The ratio of tetrahydroindene/dicyclopentadiene (by weight) in the liquid starting material is adjusted to 5/100. A part of the liquid phase in the decomposition reactor is continuously discharged out of the system so as to keep constant the quantity of liquid in the decomposition reactor. The gaseous reaction product formed in the decomposition reactor is recovered, fed to the first distillation column, and rectified at the same column top pressure, column bottom pressure and reflux ratio as in Example 4. Fraction (B-1) composed mainly of cyclopentadiene is obtained from the column top, and fraction (B-2) composed mainly of dicyclopentadiene and tetrahydroindene is obtained from the column bottom. The fraction (B-2) is discharged out of the system. As a result, the recovery rate of cyclopentadiene (the quantity of cyclopentadiene (parts by weight) in the fraction (B-1) per 100 parts by weight of dicyclopentadiene in the liquid starting material) is 74.0%, and the recovery rate of cyclopentadiene and dicyclopentadiene (total quantity (parts by weight) of cyclopentadiene and dicyclopentadiene in the fraction (B-1) per 100 parts by weight of dicyclopentadiene in the liquid starting material) is 75.0%.