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 Follistatin-like 3 (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.
