NEW MATERIAL:A compound expressed by formula I (R is H or lower alkyl).

EXAMPLE: 1-(Methoxycarbonyl)bicycle[4.4.0]deca-4,6-diene-3,8-dione.

USE: A medicine and agricultural chemical or an intermediate there having antimicrobial action and hypotensive action and raw material for electrically conductive high polymers.

PREPARATION: A bicyclo compound expressed by formula II is refluxed in the presence of water in a reaction system of an acid, e.g. methanol, while heating or a diketone compound expressed by formula III is treated with an acid, e.g. methanol, to give the aimed compound expressed by formula I industrially and advantageously, in good purity and yield.