To reduce costs by carrying out position detection of both lateral sides of a color thermal recording paper in a state while two line sensors of mutually different output levels are adjusted to be respective optimum output levels, and also by reducing a necessary memory capacity.

In a printer 1 which forms a color image on the color thermal recording paper 2, before the image formation is started, the two line sensors 17 are separately adjusted only once so that the output levels become optimum values, in a state while the color thermal recording paper 2 is stopped to be opposite to the line sensors 17. Positions of both lateral sides of the color thermal recording paper 2 are detected in a high state of S/N. A paper width of the color thermal recording paper 2 is obtained by operations and is stored. During the image formation of yellow, magenta and cyan, the position of one lateral side of the color thermal recording paper 2 is detected by using only one line sensor 17, and a heatable range among a minute heating part array of a thermal head 6 is determined on the basis of the detected position of the lateral side and the stored paper width.