In a color thermal printer, a thermal head has an array of heating elements, arranged in a main scan direction, for thermal recording to respectively pixels on thermosensitive recording material. The recording material is moved relative to the thermal head in a sub scan direction crosswise to the main scan direction, to record an image to the recording material according to image data. The recording material includes two lateral edges, opposed to each other, for extending in the sub scan direction. In the thermal printer, the image is recorded in a region extending to the two lateral edges on the recording material according to the image data. Density of two edge portions of the image disposed along the two lateral edges are lowered by processing the image data, to prevent occurrence of scorch on the two lateral edges.