Uracil derivatives and antitumor effect potentiator and antitumor agent containing the same

method for potentiating an antitumor effect of an antitumor agent containing a 2′deoxypyrmidine nucleoside, which comprises administering to a patient an effective amount of an uracil derivative represented by the following formula (1):

• wherein R1 is selected from the group consisting of chlorine, bromine, iodine, cyano and a lower alkyl group;
• and R2 represents an amino group, one or both of the hydrogen atoms on the nitrogen atom of which may each be substituted by a lower alkyl group;
• or a salt thereof.