A built-in antenna having: a support body that has a top surface shape having a flat plane that has three neighboring sides with two corners defined therebetween and inclined planes that extend downward from the respective three sides and has other planes between the inclined planes; and a metal conductor that is mounted on the support body, the metal conductor having a shape corresponding to the top surface shape of the support body, the shape being formed by bending a metal plate. The metal conductor has a first slit located corresponding to a middle side of the three sides and a second slit located extending downward from one of the two corners, the second slit being communicated with the first slit and having an opened end.