Provided are: 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.
