An antenna device is disclosed that includes a dielectric substrate having first and second surfaces facing away from each other, an element pattern formed on the first surface of the dielectric substrate, a conductive pattern formed on the first surface of the dielectric substrate so as to extend from the feeding point of the element pattern, and a ground pattern formed on the second surface of the dielectric substrate so as to form a microstrip line in cooperation with the conductive pattern. The ground pattern has a cutout part formed in a portion thereof opposing the feeding point.