To provide a solar energy-utilizing heating system to sufficiently derive the heat of a heating medium by a compact radiator.

A solar energy collector 30 to heat a heating medium by collecting solar energy is installed outdoors. Meanwhile, a fan coil unit 31 (a radiator) is installed in a closed underfloor space 1b formed by a foundation 10 and a floor material 11. By circulating the heating medium between the fan coil unit 31 and the solar energy collector 30, the fan coil unit 31 derives the heat of the heating medium and discharged the heat in the underfloor space 1b. The discharged heat is absorbed by the floor material 11 to effect heating of the interior 1a of a room.