Provided are a means for the prevention and treatment of obesity and/or insulin resistance and, particularly, pharmaceutical drugs for the treatment of obesity and/or insulin resistance under the influence of FSTL3. Specifically, provided is an insulin resistance improving drug comprising an FSTL3 inhibitor as an active ingredient, particularly, the insulin resistance improving drug, wherein the FSTL3 inhibitor is one of (A) a substance specifically binding to FSTL3 to inhibit or suppress a function of FSTL3, (B) an inhibitor for expression of FSTL3, and (C) a competitor of FSTL3.
