The present invention relates to heterocyclic compounds represented by the formula I or pharmaceutically acceptable salts thereof and antitumor agents containing the heterocyclic compounds as effective components: wherein X represents nitrogen atom or CH; R1 represents CHnF3-n (wherein n is 1 or 2), hydroxy C1–C6 alkyl, NHR6 [wherein R6 represents hydrogen atom or COR (wherein R represents hydrogen atom, C1–C6 alkyl or C1–C6 alkoxy)]; R2 represents morpholino (which may be substituted with one to four C1–C6 alkyl), thiomorpholino, piperidino, pyrrolidinyl (which may be substituted with hydroxy C1–C6 alkyl), oxazolidinyl (which may be substituted with one or two C1–C6 alkyl) or tetrahydro-1,4-thiazin-1-oxo-4-yl; R3 and R4 each represent hydrogen atom or C1–C6 alkyl; and R5 represents hydrogen atom, amino or hydroxyl.