A vehicle-installed radar sensor system includes a transmission section, a reception section, and a shield member. The transmission section is disposed in a closed space formed between a permeable member provided in a vehicle and a vehicle body, and transmits a radar wave to outside of the vehicle through the permeable member. The reception section is disposed in the closed space at a distance from the transmission section, and receives a reflected wave by a target through the permeable member. The shield member is disposed in at least one of the transmission section and the reception section, and suppresses at least one of occurrence of a reflected wave by the permeable member and reception of the reflected wave by the permeable member.