To provide a production method by which a diketonato vanadium (III) complex useful as a polymerization catalyst for a high-performance polypropylene and an ethylene-propylene copolymer, and a hardening accelerator is produced in a high yield.

This method for producing a diketonato vanadium (III) complex of the formula (R1, R2 and R3 are each a 1-8C alkyl, an aryl or H) by reacting a diketonato alkali metal salt with a vanadium salt (a water-soluble trivalent salt such as VCl3) comprises carrying out the reaction by using the diketonato alkali metal salt in an amount of 0.7-1.2 molar times as much as a theoretical equivalent at a temp. of 0-60°C, and regulating the pH of a reaction liquid after finishing the reaction so as to be 2.0-5.0.