A thermal recording system for recording an image on a thermo-sensitive recording medium using a laser beam under a preheated condition of the thermo-sensitive recording medium. In the thermal recording system, a preheating device preheats the thermo-sensitive recording medium which has leuco dyes, a developer and light-absorbing dyes all provided on a support, and which develops color in a continuous tone at a density corresponding to the energy of the applied heat, to a predetermined temperature less than a color-developing temperature. A heating-beam generating device allows a laser beam to scan the thermo-sensitive recording medium so as to heat the thermo-sensitive recording medium to a predetermined color-developing temperature. Thus, the heating-beam generating device can control the laser beam in a sufficient control range, thereby making it possible to record an image on the thermo-sensitive recording medium with high accuracy.