The display control unit 14 constructs color component image data that are used to make the image display unit 11 display an image. The image displayed by the image display unit 11 is formed by combining color component images based on color component image data. For example, the component image database 16 stores component image data of numerals 0, 1 to 9, symbols representing units, and the like. The display control unit 14 displays the color component image (as indicated by reference characters 61 to 63 in FIG. 3) in a prescribed coordinate position on the screen by using color component image data (as shown in FIG. 7, which will be described later) constructed by component image data (as shown in FIGS. 4 and 6, which will be described later) and a color palette (as shown in FIGS. 5 and 10(e), which will be described later) converted to a user-designated tint. In that way, the image display unit 11 displays the present set temperature (a combination of numeral from 0 to 99 and a unit, for example) to the user.