[Problem] To provide a treatment-resistance reducing agent for treatment-resistant cancer, said treatment-resistance reducing agent being capable of eliminating or reducing treatment-resistance, in particular, drug-resistance of cancer and thus enhancing the efficacy of cancer treatment. [Solution] A treatment-resistance reducing agent for treatment-resistant cancer that comprises, as active ingredient(s), a substance capable of specifically inhibiting the binding of interleukin-34 to colony stimulating factor-1 receptor and/or a substance capable of suppressing the expression of interleukin-34 in a cancer cell. According to the present invention, the treatment resistance of cancer, which is resistant to a treatment method such as the administration of an anticancer agent or radiation irradiation, can be reduced and, therefore, the efficacy of cancer treatment can be enhanced and the occurrence risk of a side effect can be suppressed.
