An image building apparatus of the type in which both a sheet of printing medium and a strip of ink donor medium are displaced in the superimposed state in an area defined between a printing head and a platen. The printing head thermally transfers a coloring agent on the ink donor medium to the printing medium. The width of ink donor medium is fabricated to be less than the width of printing medium. A pair of retaining members are disposed at both ends of the printing medium as seen in the transverse direction whereby the exposed parts of the printing medium at both the ends thereof are brought in pressure contract with the platen under the effect of thrusting force imparted by the retaining members.