The invention provides a novel compound having an excellent Aurora-A-selective inhibitory activity and useful as an anti-cancer agent that can be administered orally； a novel anti-tumor effect enhancer for a microtube agonist comprising a taxane-type anti-cancer agent； and a combination therapy. A piperidine compound represented by general formula (I) (wherein R1 represents a carboxyl group, -C(=O)NR5R6, or an oxadiazolyl group which may have a C1-C6 alkyl group or a trifluoromethyl group as a substituent； R2 represents a halogen atom, or a C1-C6 alkoxy group； R3 represents a phenyl group which may have 1 to 3 groups independently selected from a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group and a trifluoromethyl group as a substituent or substituents； R4 represents a hydrogen atom, or a C1-C6 alkyl group； and R5 and R6 may be the same as or different from each other and independently represent a hydrogen atom, a C1-C6 alkyl group, or a C3-C6 cycloalkyl group, or R5, R6 and a nitrogen atom to which R5 and R6 are bound may together form a 3- to 6-membered nitrogenated saturated heterocyclic group) or a salt thereof.本發明提供一種具有優異之Aurora A選擇性抑制作用、用作可經口投予之抗癌劑的新穎化合物，進而提供一種含有紫杉烷系抗癌劑之微管促效劑的新穎抗腫瘤效果增強劑及併用療法。本發明係一種哌啶化合物或其鹽，該哌啶化合物係通式(I)所示者(式中，R1表示羧基、-C(=O)NR5R6、或可具有C1-C6烷基或三氟甲基作為取代基之二唑基；R2表示鹵素原子或C1-C6烷氧基；R3表示可具有1~3個相同或不同之選自鹵素原子、C1-C6烷基、C1-C6烷氧基及三氟甲基中之基作為取代基的苯基；R4表示氫原子或C1-C6烷基；R5及R6相同或不同，表示氫原子、C1-C6烷基或C3-C6環烷基，或者R5及R6亦可與所鍵結之氮原子一併形成3~6員之含氮飽和雜環基)。
