In order to implement an external antenna of a radio device with a simple configuration, an antenna according to the present invention includes: a conductor plate; a first linear conductor including a portion disposed substantially in parallel to an edge of the conductor plate, and being curved halfway and connected to the conductor plate with a gap being interposed; a second linear conductor intersecting with a line extending from an end of the first linear conductor opposite to a connection end thereof connected to the conductor plate, and being connected to the conductor plate; a third linear conductor being connected to an end of the second linear conductor opposite to an end thereof connected to the conductor plate, and being disposed substantially in parallel to the first linear conductor; and a fourth linear conductor being disposed in a direction apart from the conductor plate at an end of the third linear conductor opposite to an end thereof connected to the second linear conductor.