To improve the degree of freedom in attachment of an ignition switch apparatus, and to make the ignition switch apparatus less likely to be influenced by heat from an engine and by electrical noise interference, an ignition switch apparatus is provided including an antenna which performs radio communication for authentication with a transponder built in a key. The ignition switch is operatively attached to a pivot plate of the vehicle frame, on which a rear wheel is supported. In order to assure good communication with the key, the antenna is situated such that it projects from an outer face of a recessed portion of the pivot plate.