PURPOSE: To provide the subject platinum complex in a short time by suspending cis-dichloro(2-aminomethylpyrrolidine) platinum (II) or an optically active isomer thereof in water and subsequently reacting the compound with 1,1- cyclobutanedicarboxylic acid silver salt.

CONSTITUTION: A compound of formula I is suspended in water and subsequently reacted with an equimolecular amount of a compound of formula II at 20-60°C for 0.5-4hr while stirring to give a compound of formula III. The compound of formula III is useful as a carcinostatic agent and can be efficiently obtained in a short time by one process.