A substrate equipped with an antenna of the present disclosure includes a circuit substrate and an antenna element. When viewed from a thickness direction, an area of one principal surface of the circuit substrate is larger than that of another principal surface thereof, and each of the one principal surface and the other principal surface of the circuit substrate is formed in a rectangular shape. When a maximum width between a first outer periphery of the other principal surface projected onto the one principal surface and a first outer periphery of the one principal surface is defined as W1, the antenna element is mounted in at least part of a region on the one principal surface of the circuit substrate, in which the region has the width W1 from the second outer periphery of the other principal surface projected onto the one principal surface toward the inner side.