To set various transparent areas on an image.

When the transparency of a transparent area and its range are designated in order to set the transparent area on the image comprising pixels having, as parameters, three channels having gradation values of a prescribed range for each of three primary colors R, G and B and an alpha channel having a gradation value of a prescribed range related to transparency, the data value Ain of the alpha channel corresponding to the designated transparency and the designated range of the transparent range are inputted (S200). The inputted data value Ain is compared with the data value A of the alpha channel set at the present for each pixel in the designated range (S202). Only the data value A of a pixel having a data value Ain smaller than the data value A (or opaque pixel) is updated to the data value Ain, and the gradation value of the corresponding pixel is calculated by use of the updated data value A (S206). The transparent area is displayed based on the calculated gradation value (S208).