Abstract: Provided is a tumor treatment method using a pyrimidine compound or a salt thereof. An antitumor agent that contains a pyrimidine compound represented by general formula (I) or a salt thereof, said antitumor agent being to be administered in combination with another antitumor agent. (In the formula, R1 represents a C1-C4 alkyl group optionally having a C1-C4 alkoxy group as a substituent, or a C3-C4 cycloalkyl group; R2 represents a hydrogen atom, a halogen atom, a C1-C6 alkyl group optionally having, as a substituent(s), one to five C1-C4 alkoxy groups or one to five fluorine atoms, or a C1-C6 alkoxy group; R3 represents a hydrogen atom, or a C1-C4 alkyl group optionally having, as a substituent(s), one to five fluorine atoms; R4 represents a hydrogen atom or a C1-C4 alkyl group; and R5 represents a phenyl group optionally having one to three substituents selected from fluorine atoms and chlorine atoms.