PURPOSE: To execute resolution to obtain a diphosphine oxide having high optical purity by bringing a racemic body of a diphosphine oxide into reaction with specific resolving agent, the reby resolving the racemic body of the diphosphine oxide to optically active antipodes.

CONSTITUTION: A racemic body of a diphosphine oxide such as (RS)-(6,6' dimethylbiphenyl2,2'-diyl)-bis(disphenylphosphine oxide) is subjected to the objective resolution by bringing resolving agents as a compd. of formula I (X is O; R1 is H, an alkyl, etc.; R2 is H, an alkyl, etc.; R3 is the remaining part of a used α-hydroxycarboxylic acid; wherein one of R1, R2 is H), a compd. of formula II (R4 is the remaining part of a used αhydroxycarboxylic acid), a compd. of formula III (R2' is an alkyl, an aryl, etc.; R5 is the remaining part of a used α-hydroxycarboxylic acid having two pieces of carboxy) and a compd. of formula IV (R6 is the remaining part of a used αhydroxycarboxylic acid having two pieces of carboxy) into reaction therewith.