PURPOSE: To provide a new 1,3-cyclopentadiene derivative having excellent carcinostatic action and practical value as a carcinostatic agent.

CONSTITUTION: The 1,3-cyclopentadiene derivative of formula I (X is H or halogen; R1 is H or 1-5C alkyl or alkoxy; R2 is H or 1-5C alkyl), e.g. 4-(4- chlorophenyl)-2-ethoxy-1,3-cyclopentadiene-l-carboxylic acid ethyl ester. The compound can be produced by reacting a compound of formula II to V (R and R' are alkyl) with a compound of formula R-CO-CH3 (R is phenyl or halogenated phenyl). When the compound is used as an active component of a carcinostatic agent, it is administered at a rate of 1mg to 10g per day in 1 to several divided doses within a range not to indicate its toxicity.