Abstract: A 5-fluorouracil derivative represented by the formula (1): ##STR1## wherein A represents a group represented by ##STR2## wherein R.sup.1 represents a hydrogen atom or OR.sup.5 group, R.sup.2, R.sup.3 and R.sup.5 may be the same or different and each represents a hydrogen atom or a group represented by the following formula (2): ##STR3## wherein R.sup.6 represents an acyl group, R.sup.7 represents a hydrogen atom, a straight or branched alkyl group, a cycloalkyl group, an aralkyl group, a lower alkenyl group or a phenyl group, and n is an integer of 0 to 6,provided that at least one of R.sup.2, R.sup.3 and R.sup.5 is a group represented by the formula (2), and the case whereinR.sup.2 and R.sup.5 are both hydrogen atoms is excluded,R.sup.4 represents a group represented by the formula (2), and X represents an alkylene group or a cycloalkylene group,and an antitumor agent containing the same as an active ingredient.