A method for treating a tumor by combining a piperidine compound of formula (I) or salt thereof and a microtubule-targeting drug, in which the microtubule-targeting drug is administered once per 7 days or more one cycle, and the piperidine compound is administered once or more per day for 4 days or more: R1 represents a carboxyl group, —C(═O)NR5R6, or an oxadiazolyl group optionally substituted with a C1-C6 alkyl group or trifluoromethyl group; R2 represents a halogen atom or C1-C6 alkoxy group; R3 represents a phenyl group optionally having 1 to 3 groups, selected from a halogen atom, and a C1-C6 alkyl, C1-C6 alkoxy, or trifluoromethyl group; R4 represents a hydrogen atom or C1-C6 alkyl group; and R5 and R6, each represent a hydrogen atom, a C1-C6 alkyl or C3-C6 cycloalkyl group; or R5 and R6, together with a nitrogen atom, optionally form a 3 to 6-membered nitrogen-containing saturated heterocyclic group.