A substrate includes, two first through-holes to which high-frequency signals are transmitted and which are arranged side by side so as to have a predetermined distance, and at least three reference potential second through-holes arranged side by side so as to have an distance smaller than the predetermined distance with respect to the two first through-holes. Among three of the second through-holes, one of the second through-holes is arranged in a region between the two first through-holes, and other two of the second through-holes are arranged in a region other than the region between the first through-holes such that one of the other two second through-holes is arranged side by side with respect to one of the two first through-holes, and the other of other two second through-holes is arranged side by side with respect to the other of the two first through-holes.