To provide a technique for modifying a waste vegetable oil, by which sodium hydroxide can be added neither too much nor too little to the waste vegetable oil.

This method for modifying the waste oil to be treated, comprising adding a methanol solution of sodium hydroxide to the oil is characterized by having a process for measuring the volume and acid value of the oil to be treated, calculating a proper sodium hydroxide addition amount necessary for modifying the oil, calculating a proper addition amount of the sodium hydroxide methanol solution from the proper sodium hydroxide addition amount and the concentration of the sodium hydroxide methanol solution, and then adding the proper sodium hydroxide addition amount of the sodium hydroxide methanol solution, and a process for calculating the proper addition amount of the methanol from the measured volume of the oil to be treated, subtracting the amount of the methanol in the added sodium hydroxide methanol solution from the proper addition amount, and then adding the calculated insufficient methanol amount to the oil to be treated.