To obtain the subject new compound useful for increasing the sensitivity of oncocytes resistant to anticancer chemotherapeutic agents and for promoting the sensitivity of multidrug-resistant cells.

This new compound (or medicinally permissible salt thereof, ester thereof or prodrug therefrom) is shown by the formula (R1 is a (substituted) 1-11C alkyl or (substituted) 2-11C alkenyl, the substituent being H, hydroxy, halo, a 1-6C alkyloxy, etc.; R2 and R3 are each an aryl, etc.; R4 is H, a (substituted) 1-11C alkyl or (substituted) 2-11C alkenyl, the substituent being H, hydroxy, halo, a 1-6C alkyloxy, etc.), e.g. 2-(4-hydroxyphenyl)-1,4,5-triphenyl- imidazoletrifluoroacetic acid salt. This compound of the formula can be prepared advantageously by using readily available starting substance(s) through solid phase synthesis or solution phase synthesis, and is used through administration of sufficient amounts thereof before, during or after administration of an antitumor chemotherapeutic agent.