A radio system includes: a radio device; an antenna device; and a polarization converting circuit for changing the directions of polarized waves, inserted at a position between the radio device and the antenna device. The polarization converting circuit is installed inside a holder in a device interface portion, and the device interface portion has a through hole that penetrates from the outer wall of the device interface portion to the outer periphery of the polarization converting circuit and is used to confirm the rotating angle of the polarization converting circuit inside the holder.