Abstract: Provided is a thermosetting resin composition that can satisfy both low transmission loss and radiation performance even under the same curing conditions as those for conventional substrate materials for high-frequency applications, and also provided are an adhesive sheet, a prepreg, and a laminate. The thermosetting resin composition comprises a maleimide compound having at least two maleimide groups per molecule, a polyphenylene ether compound having at least two reactive organic groups per molecule, a curing accelerator, and an inorganic filler, wherein the inorganic filler is low-sodium aluminum oxide, and the low-sodium aluminum oxide has an Na+ ion content of 10 ppm or less.