A mode coding unit for an image compression device includes a submode determination unit, a variable bit determination unit, and a coding unit. The submode determination unit is configured to determine a submode of an image block. The submode indicates a matching degree between an upper bit of the image block and an upper bit of a reference block. The variable bit determination unit is configured to determine a size of primary color compression information fields corresponding to primary colors in a reference pixel in the image block based on a relational size of color information of the primary colors. The coding unit is configured to generate compressed image data by coding the image block based on the submode and the size of the primary color compression information fields.