Non-thermal radio emissions from a hot Jupiter and planet-modulated chromospheric activities of solar-type stars reported the tentative detection of a planetary magnetic field. The magnetosphere of a young gas giant affects the structure of a cirumplanetary disk in which satellite formation occurs. Also, the global magnetic field of a planet is closely related to the internal compositions and the thermal state. Toward the future of the detection of planetary magnetic fields, such as the era of the Square Kilometer Array, we examine the relationship between the magnetic field in an extrasolar giant and the interior. A more massive extrasolar gas giant has a stronger magnetic field over Gyrs. We find that no dynamo-driven magnetic field emerges in a hot Saturn with no core or a small one until 10-100 Myr after its formation, whereas the magnetic field strength of a hot Jupiter is insensitive to its interior structure. Detecting the magnetic field of a young, hot Saturn is a good indicator of the existence of an elusive core, although it may be challenging because of the weakness of radio signals and the shielding effect of plasma in the Earth’s ionosphere. Hot Jupiters with > 0.4 times Jupiter-mass are suitable for the search for radio emissions from exoplanets.