A novel compound which has an excellent aurora A-selective inhibitory action and is useful as an orally administrable anticancer agent is provided. Also, a novel agent for potentiation of anti-tumor effect of microtubule agonists, which include a taxane anticancer agent, and a combination therapy are provided. A piperidine compound represented by a general formula (I) or a salt thereof: wherein, R1 represents a carboxyl group, —C(═O)NR5R6, or an oxadiazolyl group optionally having 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 optionally having 1 to 3 same or different group(s) selected from a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, and a trifluoromethyl group as a substituent; R4 represents a hydrogen atom or a C1-C6 alkyl group; and R5 and R6 are the same or different and each represent a hydrogen atom, a C1-C6 alkyl group, or a C3-C6 cycloalkyl group, or R5 and R6 optionally form a 3 to 6-membered nitrogen-containing saturated heterocyclic group together with a nitrogen atom to which R5 and R6 are bound, are provided.