PROBLEM TO BE SOLVED: To provide an agent for reducing therapy resistance to therapy-resistant cancer that can resolve or reduce therapy resistance of cancer, particularly drug resistance, and can increase effectiveness of cancer therapy.SOLUTION: The present invention relates to an agent for reducing therapy resistance of therapy-resistant cancer which has a substance specifically inhibiting a bond between interleukin-34 and a colony stimulating factor-1 receptor and/or a substance inhibiting the expression of interleukin-34 in a cancer cell as active ingredients. According to the present invention, therapy resistance of cancer which shows resistance to a therapy such as administration of an anticancer agent and radiation exposure can be reduced, effectiveness in cancer therapy can be increased, and generation of an adverse effect or the like can be suppressed.SELECTED DRAWING: Figure 7
