To provide an antitumor agent which exhibits excellent c-Met inhibitory effect and which mitigates adverse effects by virtue of high selective effect to tumor cells in which c-Met is specifically expressed. The invention provides an acylthiourea compound represented by formula (I):(wherein each of R1and R2, which may be identical to or different from each other, represents a hydrogen atom, an optionally substituted C1-6alkyl group, an optionally substituted C3-10cycloalkyl group, an optionally substituted C6-14aromatic hydrocarbon group, or an optionally substituted saturated or unsaturated heterocyclic group, or R1and R2may form, together with the nitrogen atom to which they are bonded, an optionally substituted nitrogen heterocyclic ring； R3represents a C1-6alkyl group； and each of R4, R5, and R6, which may be identical to or different from one another, represents a hydrogen atom, a halogen atom, an optionally substituted C1-6alkyl group, a C1-6alkoxy group, a C1-6alkylamino group, an optionally substituted aromatic hydrocarbon group, or an optionally substituted saturated or unsaturated heterocyclic group, or R5and R6may form a ring together with the phenyl ring to which they are bonded) or a salt thereof.本發明提供一種藉由具有優異之c-Met抑制作用，且選擇性地作用於c-Met特異表現之腫瘤細胞，從而減輕副作用的抗腫瘤劑。本發明提供一種通式(I)所表示之醯基硫脲化合物或其鹽：[式中，R1及R2相同或不同，表示氫原子、可具有取代基之C1-6烷基、可具有取代基之C3-10環烷基、可具有取代基之C6-14芳香族烴基、或可具有取代基之飽和或不飽和雜環基，或者R1及R2可與所鍵結之氮原子一起形成可具有取代基之含氮雜環；R3表示C1-6烷基；R4、R5及R6相同或不同，表示氫原子、鹵素原子、可具有取代基之C1-6烷基、可具有取代基之C1-6烷氧基、可具有取代基之C1-6烷基胺基、可具有取代基之芳香族烴基、或可具有取代基之飽和或不飽和雜環基，或者R5及R6可與所鍵結之苯環一起形成環]。
