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, R 1 represents a carboxyl group, -C(=O)NR 5 R 6 , or an oxadiazolyl group optionally having a C 1 -C 6 alkyl group or a trifluoromethyl group as a substituent R 2 represents a halogen atom or a C 1 -C 6 alkoxy group R 3 represents a phenyl group optionally having 1 to 3 same or different group(s) selected from a halogen atom, a C 1 -C 6 alkyl group, a C 1 -C 6 alkoxy group, and a trifluoromethyl group as a substituent R 4 represents a hydrogen atom or a C 1 -C 6 alkyl group and R 5 and R 6 are the same or different and each represent a hydrogen atom, a C 1 -C 6 alkyl group, or a C 3 -C 6 cycloalkyl group, or R 5 and R 6 optionally form a 3 to 6-membered nitrogen-containing saturated heterocyclic group together with a nitrogen atom to which R 5 and R 6 are bound, are provided.
