PURPOSE: To obtain a liquid diphenylmethane diisocyanate having sstabilized properties by adding a phosphorin-based catalyst and a phosphorous ester to diphenylmethane diisocyanate and carrying out the reaction.

CONSTITUTION: The method for producing a liquid diphenylmethane diisocyanate (MDI) is to heat the MDI into a liquid form, add a phosphorin-based catalyst (e.g. 1-methylphosphorin-1-oxide) and a phosphorous ester of formula I or II (R1 to R7 are each a 4-20C alkyl) thereto and carry out the carbodiimidating reaction at 60-80°C for 1-10hr. The liquid MDI is not colored even in storage for a long period. The MDI is extremely rich in reactivity and useful as a raw material for polyurethane resins obtained by reaction thereof with a polyol mixture prepared by adding a cross-linking agent, a catalyst, etc., to an active hydrogen compound. The amounts of the raw materials based on 100 pts.wt. MDI are 0.0001-0.005 pt.wt. catalyst 0.001-0.1 pt.wt. hypophosphorous ester, i.e., several tens to hundreds of times the amount of the added catalyst.