PURPOSE: To realize uniform irradiation and to reduce the size of a fixed frame, by moving a light source by a link mechanism which constitutes the approximately line motion mechanism of Roberts' linkage in a thermal copying machine.

CONSTITUTION: An infrared-ray transmissive base plate 13 where an original 16 and heat-sensitive recording paper 17 are mounted is provided at the upper part of the external fixed frame body 12, and longitudinal links 1A and 1B constituting the approximately linear motion mechanism of the Roberts' linkage and a driving link member 5 having a gear part 6 are coupled by a triangular light source base in a drawing; and a reflecting mirror 10 and the light source 11 are provided on the light source base. When rotating force is applied to the gear part 6 by a driving part 8, the respective link members are supported pivotally at fixing part 2AW2C and the light source is in approximately linear motion. consequently, the uniform irradiation is obtained and the fixed frame body 12 is reduced in size to area nearly equal to the effective area of the base plate 13.