A cyan thermal coloring layer, a magenta thermal coloring layer, and a yellow thermal coloring layer are successively formed on a base of a thermo-sensitive recording paper. A yellow thermal head, a magenta thermal head, and a cyan thermal head are disposed along a conveyance path. While the thermo-sensitive recording paper is being conveyed in the sub scanning direction along the conveyance path, three colors images are sequentially recorded by the three thermal heads. When the length of heat elements of the respective thermal heads are Ly, Lm and Lc in a sub scanning direction, it follows that Ly